1.Read only such books _ you can understand

1.Read only such books _ you can understand
A.as B.that C.what D.which
正确答案为A,为什么不选B呢?
2._it is to jump into a river in summer!
A.How fun B.How funny C.What great fun D.What a great fun
正确答案为C,为什么不选A呢?
that在这里做用在定语从句中不行吗?
how fun 一般在句尾用is it
What great fun 用 it is
为什么是这样的呢?