diff --git a/option/credentials_go19.go b/option/credentials_go19.go
deleted file mode 100644
index 2e3d2dedbbf..00000000000
--- a/option/credentials_go19.go
+++ /dev/null
@@ -1,24 +0,0 @@
-// Copyright 2018 Google LLC.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build go1.9
-// +build go1.9
-
-package option
-
-import (
-	"golang.org/x/oauth2/google"
-	"google.golang.org/api/internal"
-)
-
-type withCreds google.Credentials
-
-func (w *withCreds) Apply(o *internal.DialSettings) {
-	o.Credentials = (*google.Credentials)(w)
-}
-
-// WithCredentials returns a ClientOption that authenticates API calls.
-func WithCredentials(creds *google.Credentials) ClientOption {
-	return (*withCreds)(creds)
-}
diff --git a/option/credentials_notgo19.go b/option/credentials_notgo19.go
deleted file mode 100644
index 728a62f1ae6..00000000000
--- a/option/credentials_notgo19.go
+++ /dev/null
@@ -1,23 +0,0 @@
-// Copyright 2018 Google LLC.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build !go1.9
-// +build !go1.9
-
-package option
-
-import (
-	"golang.org/x/oauth2/google"
-	"google.golang.org/api/internal"
-)
-
-type withCreds google.DefaultCredentials
-
-func (w *withCreds) Apply(o *internal.DialSettings) {
-	o.Credentials = (*google.DefaultCredentials)(w)
-}
-
-func WithCredentials(creds *google.DefaultCredentials) ClientOption {
-	return (*withCreds)(creds)
-}
diff --git a/option/option.go b/option/option.go
index 9ff697e0b8e..762cf5ae39b 100644
--- a/option/option.go
+++ b/option/option.go
@@ -10,6 +10,7 @@ import (
 	"net/http"
 
 	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/google"
 	"google.golang.org/api/internal"
 	"google.golang.org/api/internal/impersonate"
 	"google.golang.org/grpc"
@@ -328,3 +329,14 @@ func (i impersonateServiceAccount) Apply(o *internal.DialSettings) {
 	o.ImpersonationConfig.Delegates = make([]string, len(i.delegates))
 	copy(o.ImpersonationConfig.Delegates, i.delegates)
 }
+
+type withCreds google.Credentials
+
+func (w *withCreds) Apply(o *internal.DialSettings) {
+	o.Credentials = (*google.Credentials)(w)
+}
+
+// WithCredentials returns a ClientOption that authenticates API calls.
+func WithCredentials(creds *google.Credentials) ClientOption {
+	return (*withCreds)(creds)
+}
diff --git a/transport/dial.go b/transport/dial.go
index 2c495ad5389..652b8eba51d 100644
--- a/transport/dial.go
+++ b/transport/dial.go
@@ -8,8 +8,10 @@ import (
 	"context"
 	"net/http"
 
+	"golang.org/x/oauth2/google"
 	"google.golang.org/grpc"
 
+	"google.golang.org/api/internal"
 	"google.golang.org/api/option"
 	gtransport "google.golang.org/api/transport/grpc"
 	htransport "google.golang.org/api/transport/http"
@@ -34,3 +36,13 @@ func DialGRPC(ctx context.Context, opts ...option.ClientOption) (*grpc.ClientCon
 func DialGRPCInsecure(ctx context.Context, opts ...option.ClientOption) (*grpc.ClientConn, error) {
 	return gtransport.DialInsecure(ctx, opts...)
 }
+
+// Creds constructs a google.Credentials from the information in the options,
+// or obtains the default credentials in the same way as google.FindDefaultCredentials.
+func Creds(ctx context.Context, opts ...option.ClientOption) (*google.Credentials, error) {
+	var ds internal.DialSettings
+	for _, opt := range opts {
+		opt.Apply(&ds)
+	}
+	return internal.Creds(ctx, &ds)
+}
diff --git a/transport/go19.go b/transport/go19.go
deleted file mode 100644
index d40bcf3325f..00000000000
--- a/transport/go19.go
+++ /dev/null
@@ -1,26 +0,0 @@
-// Copyright 2018 Google LLC.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build go1.9
-// +build go1.9
-
-package transport
-
-import (
-	"context"
-
-	"golang.org/x/oauth2/google"
-	"google.golang.org/api/internal"
-	"google.golang.org/api/option"
-)
-
-// Creds constructs a google.Credentials from the information in the options,
-// or obtains the default credentials in the same way as google.FindDefaultCredentials.
-func Creds(ctx context.Context, opts ...option.ClientOption) (*google.Credentials, error) {
-	var ds internal.DialSettings
-	for _, opt := range opts {
-		opt.Apply(&ds)
-	}
-	return internal.Creds(ctx, &ds)
-}
diff --git a/transport/http/default_transport_go113.go b/transport/http/default_transport_go113.go
deleted file mode 100644
index 69b2280cddc..00000000000
--- a/transport/http/default_transport_go113.go
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright 2020 Google LLC.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build go1.13
-// +build go1.13
-
-package http
-
-import "net/http"
-
-// clonedTransport returns the given RoundTripper as a cloned *http.Transport.
-// It returns nil if the RoundTripper can't be cloned or coerced to
-// *http.Transport.
-func clonedTransport(rt http.RoundTripper) *http.Transport {
-	t, ok := rt.(*http.Transport)
-	if !ok {
-		return nil
-	}
-	return t.Clone()
-}
diff --git a/transport/http/default_transport_not_go113.go b/transport/http/default_transport_not_go113.go
deleted file mode 100644
index 90ee1bd6185..00000000000
--- a/transport/http/default_transport_not_go113.go
+++ /dev/null
@@ -1,16 +0,0 @@
-// Copyright 2020 Google LLC.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build !go1.13
-// +build !go1.13
-
-package http
-
-import "net/http"
-
-// clonedTransport returns the given RoundTripper as a cloned *http.Transport.
-// For versions of Go <1.13, this is not supported, so return nil.
-func clonedTransport(rt http.RoundTripper) *http.Transport {
-	return nil
-}
diff --git a/transport/http/dial.go b/transport/http/dial.go
index 179534a0c7c..cab709f0c05 100644
--- a/transport/http/dial.go
+++ b/transport/http/dial.go
@@ -210,3 +210,14 @@ func addOCTransport(trans http.RoundTripper, settings *internal.DialSettings) ht
 		Propagation: &propagation.HTTPFormat{},
 	}
 }
+
+// clonedTransport returns the given RoundTripper as a cloned *http.Transport.
+// It returns nil if the RoundTripper can't be cloned or coerced to
+// *http.Transport.
+func clonedTransport(rt http.RoundTripper) *http.Transport {
+	t, ok := rt.(*http.Transport)
+	if !ok {
+		return nil
+	}
+	return t.Clone()
+}
diff --git a/transport/not_go19.go b/transport/not_go19.go
deleted file mode 100644
index bb5667a5199..00000000000
--- a/transport/not_go19.go
+++ /dev/null
@@ -1,26 +0,0 @@
-// Copyright 2018 Google LLC.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build !go1.9
-// +build !go1.9
-
-package transport
-
-import (
-	"context"
-
-	"golang.org/x/oauth2/google"
-	"google.golang.org/api/internal"
-	"google.golang.org/api/option"
-)
-
-// Creds constructs a google.DefaultCredentials from the information in the options,
-// or obtains the default credentials in the same way as google.FindDefaultCredentials.
-func Creds(ctx context.Context, opts ...option.ClientOption) (*google.DefaultCredentials, error) {
-	var ds internal.DialSettings
-	for _, opt := range opts {
-		opt.Apply(&ds)
-	}
-	return internal.Creds(ctx, &ds)
-}