7 Fragment可以实现动态加载，下面是实现动态加载的过程，请补全代码？private void replaceFragment(int layout,Fragment fragment){ FragmentManager manager=getSupportFragmentManager(); FragmentTransaction fragmentTransaction=manager.beginTransaction(); fragmentTransaction.replace(layout,fragment); fragmentTransaction.addToBackStack(null); ( ？ ); } A: fragmentTransaction.getFragmentById() B: fragmentTransaction.getAcitivity() C: fragmentTransaction.commitFragment() D: fragmentTransaction.commit()