无法删除片段

Question

我无法从fragmentTransaction中删除片段。我想删除我在oncreate方法中添加的片段。以下是我的代码：

fragmentManager = getSupportFragmentManager();
            fragmentTransaction = fragmentManager.beginTransaction();
            firstFragment = new FirstFragment();
            secondFragment = new SecondFragment();

            fragmentTransaction
                    .add(R.id.container_first, firstFragment)
                    .add(R.id.container_second, secondFragment)
                    .commit();


            VideoView video = (VideoView) findViewById(R.id.videoView1);

            video.setVideoURI(Uri.parse("http://192.168.43.10/video.mp4"));
            video.start();


            Button btn = (Button) findViewById(R.id.button1);
            btn.setOnClickListener(new OnClickListener(){

                @Override
                public void onClick(View arg0) {
                    Toast.makeText(MainActivity.this, "im here", 1500).show();



                    FragmentTransaction ft = fragmentManager.beginTransaction();    

                        ft.remove(secondFragment);
                        ft.commit();
                }});