几道新东方蓝皮书的问题~~~请大家指教

1 In the 1980's the rate of increase of the minority population of the United States was twice as fast as the 1970's.

A.twice as fast as

B.twice as fast as it was in

C.twice what it was in

D.two times faster than that of

E.two times greater than

KEY:B

为什么不选D？新东方老师说过要代替前面出现过的事物只能用THAT不能用IT。在所有ＧＭＡＴ的倍数表示法中two times都是一种有歧义的不好说法，它除了表两倍还可以表示两次！所以一般在选项中看到用two times表示倍数就可以直接Ｔ掉了

2 His studies of ice-polished rocks in his Alpine home-land ,far outside the range of present-day glaciers, led Louis Agassiz

in 1837 to propose the concept of an age in which great ice sheets had existed in now currently temperate areas.

A. in which great ice sheets had existed in now currently temperate areas

B.in which great ice sheets existed in what are now temperate areas

C.when great ice sheets existed where there were areas now temperate

D.when great ice sheets had existed in current temperate areas

E.when great ice sheets existed in areas now that are temperate

KEY：B

我觉得B中的WHAT ARE的表达罗嗦~~不如D选项好 

而且这题用的是WHEN 与新东方老师的讲法好象不一致~~请问各位什么时候用WHEN 什么时候用IN WHICH？

age in which是一种固定结构记住就好

3.It is as difficult to prevent crimes against property as those that are against a person.

A.those that are against a

B.those against a

C.it is against a

D.preventing those against a

E.it is to prevent those against a

KEY:E

为什么不可以选择B答案呢？偶觉得Ｂ把比较对象改变了，原句比较的是to prevent crimes against property 和to prevent those against a person如果选Ｂ变成了一个动作prevent...和一个名词crimes...的比较

希望各位大侠能帮我解答以上问题~~~先谢谢大家了！！