问题补充：
The reason he had to look after his mother at hospital was not the one he gave to us.
A. why, why
B. that, which
C. why, that
D. that, why
答案：
A. why, why
B. that, which
(答案→)C. why, that
D. that, why
解析：在…he had to look after his mother at hospital定语从句中不缺主语和宾语，所以不用that，故排除BD；the one代替The reason避免重复，在…he gave to us定语从句中缺少gave的间接宾语，why不能作宾语，故排除A，所以应选C。
