问题描述：
he worked hard _____it.
He learned by himself.He worked hard _____it.Now he is a great magician.
A.in
B.on
C.for
D.at
选什么 为什么
He learned by himself.He worked hard _____it.Now he is a great magician.
A.in
B.on
C.for
D.at
选什么 为什么
问题解答：
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