英语方面的问题.1.She usually goes to work ______ big car.A.by B.in

英语方面的问题.
1.She usually goes to work ______ big car.
A.by B.in a C.on a D.by a
我知道答案选B 可是为什么选B
2.About _______ people took part in the activities of Expo 2010 in Shanghai.
A.millions of B.16 million of C.16 millions of D.16 million
我也知道答案,选D,也是原因不知道、、、