If someone knocks and it's not an important matter, excuse yourself and let the person know you're busy so they can get the hint(暗示) that when the door is closed, you're not to bedisturbed.
如果有人敲门, 并且不是什么重要的事情, 那就礼貌地谢绝, 让别人知道你很忙, 这样他们就会得到暗示: 当门关着的时候, 你不想被打扰。
2017全国卷2