关于WHAT和that的英语题,

题目

关于WHAT和that的英语题,
what did your parents think about your decision?they always let me do ____ i think i should,A.when B.that C.how D.what 麻烦帮我解释下

答案

that、what都可以引导名词从句.但that是个连词,不能在名词从句中充当句子成分,而what除引导从句之外,还在它所引导的从句中充当句子成分（主语、表语或宾语）.
此题选D.what 因为它引导一个宾语从句（为do的宾语）,并且还在从句中作宾语,作should（do）的宾语.