Cannot resolve 'R'.. not sure why this is happening?? Android Studio HW

Question

enter image description hereOBJECTIVE: to play animals sounds of cows and pigs

1. The opening screen displays an img of a farm and the title, animal voices for six seconds

2. second screen will display img on a cow and a pig with their respective play buttons under them.

3. When one button is pressed, the other button and image disappear and vice versa.

4. when a button is pressed, their respective sounds will be played

   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);
               }
           }
       };
   }