关于hundred 的用法,哪个对?

关于hundred 的用法,哪个对?
有两个题：
_____people are sitting quitely in the hall and listeing to the speech.
A.Six hundred of; B.Six hundreds; C.Six hundred; D.Six hundreds of
So far,they have sold over three_____books.
A.million; B.millions of; C.million of; D.millions;
是D.和A.
我有点看不明白,我倾向于C和A.是我错了还是答案有问题.
还有请总结一下这种数词什么时候加s,什么时候不加.