请帮我解释下列英语问题.

请帮我解释下列英语问题.
1 He made another wonderful discovery,___of great importance to science.
A which i think is
B which i think it is
C which i think it
Di think which is
为什么选择A．
2The wolves hid themselves in the places___couldn't be found.
A that B where C in which D in that
为什么选择A
3 Is this the plane___he came to New York?
A which B by which C that D in which
为什么选择B
4 She likes to use words___is clear to him.
A of which the meaning
B of which meaning
C whose of meaning
D meaning of which
为什么选择A
5 He hend me several books,___were very good.
A five of which
B of which
C which of five
D five
为什么选择A．