Fragment后退栈的问题

[默默丶遗忘]

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    private Button btn1,btn2, btn3;
    private FragmentManager manager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btn1 = (Button) findViewById(R.id.btn1);
        btn2 = (Button) findViewById(R.id.btn2);
        btn3 = (Button) findViewById(R.id.btn3);
        btn1.setOnClickListener(this);
        btn2.setOnClickListener(this);
        btn3.setOnClickListener(this);
        manager = getFragmentManager();
        FragmentTransaction transaction = manager.beginTransaction();
        transaction.add(R.id.ll, new FragmentA());
        transaction.commit();
    }

    @Override
    public void onClick(View v) {
        FragmentTransaction transaction = manager.beginTransaction();
        switch (v.getId()) {
            case R.id.btn1:
                transaction.replace(R.id.ll, new FragmentA());
                break;
            case R.id.btn2:
                transaction.replace(R.id.ll, new FragmentB());
                break;
            case R.id.btn3:
                transaction.replace(R.id.ll, new FragmentC());
                break;

        }
        transaction.addToBackStack(null);
        transaction.commit();

    }
}

加入后退栈后,按返回还是直接返回了??有没有大神解释下

FragmentA类的代码如下

public class FragmentA extends Fragment {

    public TextView tv_content;
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.item, container, false);
         tv_content= (TextView) view.findViewById(R.id.tv_content);
        setTv_content();
        return view;
    }
    public void setTv_content() {
        tv_content.setText("新闻");
    }
}

B类和C类是直接继承A类 重写SetTV_Content方法的...