一道英语选择题，为什么用what而不用that,which，what常在什么情况下用？

Question

Llooking back ,the old man always feels proud of ＿he was achieved—a happy family.
A.which B.what C.where D.that

Answer 1

what没错。what这个词，有两个作用：一要起连接，二要在从句中充当成份。而that, which在名词性从句中只起连接作用。这里是在of后，又没有先行词，判断为名词性从句。故你填的词应满足：一是起连接，二是充当从句中achieved的宾语，只有what