关于高中定语定语从句的问题I shall remember the day ______ we met firstA、t

关于高中定语定语从句的问题
I shall remember the day ______ we met first
A、that
B、which
C、where
D、when
为什么不能选A
It was raining,______ was a pity.
A、what
B、that
C、which
D、that which
He studied hard at school when he was young ______ contributes to his success in later life
A、as
B、therefore
C、which
D、so that
CThe size of the audience,______ we had expected,was well over one thousand.
A、whom
B、who
C、as
D、that
He failed his exam; _______ proves that he wasn’t working hard enough
A、that
B、which
C、such
D、what
青帮我分析下每个选项,