1． The book was written in 1946,_D___the education system has witnessed great changes.
A. When
B. During which
C. Since then
D. Since when
第一题when不是指带 in 1946?

3.when asked why he went there, he said he was sent there__D___for a space flight.
A. training
B.being trained
C.to have trained
D.to be trained
C.to have trained 为什么不对？

第一题，which指代1946.

定语从句我们常用还原从句法。还原后是：

During 1946 the education system has witnessed great changes.

第三题，to have done，是非谓语问题，

非谓语当中，有这样一条约定俗成的规律，就是，to be doing 和 to have done ，是to do的结构的时态变形，只能用于固定搭配结构中。比如： be said to do 是“据说...”，而 be said to have done “据说已经...”，表示这儿动作发生在据说之前。

而 to do 结构本身，表示这个动作“要做，还未做，目的性”。

所以要明确 to do 和 to have done 的区别。

本题中，比较容易区分。他说他被派往那里的目的是 .... 所以是一般性的目的，不含有已经的含义。所以答案只能是D。