I、单项选择：从A、B、C、D四个选项中，选出最佳选项。
1、He wanted to know the reason ___ I was late.
A. as B. for C. why D. because
2、The sun heats the earth, ___ is very important to living things.
A. that B. what C. which D. where
3、I shall remember the days ____ we studied together.
A. that B. which C. where D. when
4、There isn’t so much noise in the country ___ in big cities.
A. that B. which C. as D. what
5、Do you know who lives in the building ____ there is a well?
A. in front of it B. in front of whose
C. in front of which D. in front which
6、It is known that if a man ______ achieve much, he must try as hard as he can.
