要由D回来给A，使用意向的标志。

To Come Back from D to A, use Intent Flags.

Intent intent = new Intent(activity, activityToStart);
intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP)
startActivity(intent);

FLAG_ACTIVITY_CLEAR_TOP，会而不是创建新的活动，它会调用堆栈上的活动，并会弹出所有的活动在被调用的活动。

FLAG_ACTIVITY_CLEAR_TOP, will instead of creating new activity, it will invoke the activity on the stack, and will pop all the activities over the activity being invoked.