几道英语题,希望详细解释.谢谢啦.

几道英语题,希望详细解释.谢谢啦.
1.Down __from the tree.
A.fell it B it fell
为什么选B不选A.
2.---Oh,my God.Tom is singing again.
---He always __himself as a bit of a singer.
A.fancies B.considers.
为什么不选B选A.
3.It is ten years since the scientist__on his life work of discovering the valuable chemical.
A.made for B.set out C.took off D.turned up
为什么选B.
4.---It's twenty yeas since we graduated from the high school.
---But I still remeber the story,believe or not,__we got lost on a rainy night.
A.which B.what C.that D.when
为什么选C.
5.__that he had no qualification in business management,he planned to take up a course.
A.Concerning B.Concerned
为什么选B