The thought of going back home was ----------kept him happy while he was working abroad

问题描述:

The thought of going back home was ----------kept him happy while he was working abroad
A that
B all that
C all what
D which 为什么不选A呢 不是定语从句吗

B
表语部分意为“所.的”,所以要么是what kept him happy ,要么是all that kept him happy
因为what= all that,可见A 不对