为什么android.gsm.ads类无法识别？

Question

此代码未编译，广告类似乎存在问题。注意：我正在使用Android Studio。我该如何解决这个问题？

 package com.mycompany.whatarethose;

    import android.app.Activity;
    import android.media.AudioManager;
    import android.media.MediaPlayer;
    import android.os.Bundle;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.View;
    import android.view.View.OnClickListener;
    import android.widget.Button;
    import android.widget.Toast;

    import com.google.android.gms.ads.*;
    import com.google.android.gms.ads.AdRequest;
    import com.google.android.gms.ads.AdView;

    //Author : Ayodeji Marquis

    public class MainActivity extends Activity implements OnClickListener {

        private MediaPlayer mp;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            setVolumeControlStream(AudioManager.STREAM_MUSIC);
            Button button1=(Button)findViewById(R.id.button_1);
            button1.setOnClickListener(this);

            // Load an ad into the AdMob banner view.
            AdView adView = (AdView) findViewById(R.id.adView);
            AdRequest adRequest = new AdRequest.Builder().build();
            adView.loadAd(adRequest);
        }


        public void onClick(View v) {
            int resId;
            switch (v.getId()) {
                case R.id.button_1:
                    resId = R.raw.what;
                    break;

                default:
                    resId = R.raw.what;
                    break;
            }
            // Release any resources from previous MediaPlayer
            if (mp != null) {
                mp.release();
            }
            // Create a new MediaPlayer to play this sound
            mp = MediaPlayer.create(this, resId);
            mp.start();
        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.menu_main, menu);
            return true;
        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            // Handle action bar item clicks here. The action bar will
            // automatically handle clicks on the Home/Up button, so long
            // as you specify a parent activity in AndroidManifest.xml.
            int id = item.getItemId();

            if (id == R.id.action_settings) {
                return true;
            }

            return super.onOptionsItemSelected(item);
        }


        @Override
        protected void onDestroy() {
            if(null!=mp){
                mp.release();
            }
            super.onDestroy();
        }
    }

以下是 Build.gradle 文件以获取更多信息

apply plugin: 'com.android.application'

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.1"

    defaultConfig {
        applicationId "com.mycompany.whatarethose"
        minSdkVersion 8
        targetSdkVersion 22
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:22.1.1'
    compile 'com.google.android.gms:play-services-ads:6.5.87'
}