对话框的标题和内容都可以显示出来，为什么定义的数组列表显示不出来呢？

Question

public class SingleActivity extends Activity {

Button bt;

String aa[]={"1","2","3","4","5",};

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_single);

bt=(Button) findViewById(R.id.button1);

final AlertDialog.Builder B = new AlertDialog.Builder(SingleActivity.this);

B.setTitle("单选对话框");

B.setMessage("请选择");

B.setSingleChoiceItems(aa, 0, new DialogInterface.OnClickListener() {

@Override

public void onClick(DialogInterface arg0, int arg1) {

// TODO Auto-generated method stub

String str=aa[arg1];

Toast.makeText(SingleActivity.this, str, Toast.LENGTH_LONG);

}

});

bt.setOnClickListener(new OnClickListener() {

public void onClick(View arg0) {

// TODO Auto-generated method stub

AlertDialog ad=B.create();

ad.show();

}

});

}

}