无法解决'R'..不确定为什么会发生这种情况？ Android Studio HW

Question

enter image description here 目标：播放奶牛和猪的动物声音

1. 打开的屏幕显示一个农场的img和标题，动物的声音六秒

2. 第二个屏幕将在牛和猪上显示img，其下方有各自的播放按钮。

3. 按下一个按钮时，另一个按钮和图像消失，反之亦然。

4. 按下按钮时，将播放各自的声音

   package net.androidbootcamp.animalvoices;
   
   import android.media.MediaPlayer;
   import android.support.v7.app.AppCompatActivity;
   import android.os.Bundle;
   import android.view.View;
   import android.widget.Button;
   
   public class MainActivity extends AppCompatActivity {
       Button btnCow, btnPig;
       MediaPlayer mpPig, mpCow;
       int playing;
   
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);
           btnPig = (Button)findViewById(R.id.btnPig);
           btnCow = (Button)findViewById(R.id.btnCow);
           btnPig.setOnClickListener(bPig);
           btnCow.setOnClickListener(bCow);
           mpPig = new MediaPlayer();
           mpPig = MediaPlayer.create(this, R.raw.pigs);
           mpCow = new MediaPlayer();
           mpCow = MediaPlayer.create(this, R.raw.cows);
           playing = 0;
       }
       Button.OnClickListener bPig = new Button.OnClickListener() {
           @Override
           public void onClick(View v) {
               switch (playing) {
                   case 0:
                       mpPig.start();
                       playing = 1;
                       btnCow.setText("Pause the Cows");
                       btnPig.setVisibility(View.INVISIBLE);
                       break;
                   case 1:
                       mpPig.pause();
                       playing = 0;
                       btnCow.setText("Listen to the Cows");
                       btnPig.setVisibility(View.VISIBLE);
                       break;
               }
           }
       };
       Button.OnClickListener bCow = new Button.OnClickListener() {
           @Override
           public void onClick(View v) {
               switch (playing) {
                   case 0:
                       mpCow.start();
                       playing = 1;
                       btnCow.setText("Pause the Pigs");
                       btnPig.setVisibility(View.INVISIBLE);
                       break;
                   case 1:
                       mpCow.pause();
                       playing = 1;
                       btnCow.setText("Listen to the Pigs");
                       btnPig.setVisibility(View.VISIBLE);
               }
           }
       };
   }