由某些状语从句引出的省略

下面这道题选哪个答案? 与省略句有关吗?

He is a man of few words. He never speaks unless __________.

A. speaking                       B. spoken                     C. speaking to                D. spoken to

此题应选D。这是省略句，补完整为。如：

He is a man of few words. He never speaks unless he is spoken to. 他是一个沉默寡言的人，除非别人同他说话，否则他从不说话。

在英语中，一些表示时间、地点、条件、方式、 让步等的从句，若其主语与主句主语一致，且从句谓语包含动词be, 那么可将从句的主语和动词be省略。如：

You must study hard while (you are) young, or you will regret when (you are) old. 趁年轻时要努力学习，不然到老了你会后悔的。

I won’t go unless (I am) invited. 我不会去，除非请我。

He worked very hard though (he was) still rather poor in health. 尽管身体还不好，但他仍努力工作。

He will work hard wherever (he is) sent by the Party. 无论党把他派往哪里，他都会努力工作。

While (I was) waiting I was reading some old magazines. 等的时候我在看一些旧杂志。

If (it is) carefully done, the experiment will be successful. 如果做得仔细，这个实验会成功。

Look out for cars when (you are) crossing the streets. 过马路时要注意汽车。