问几道SAT语法题,IS题the difference between L and S is that the form

问几道SAT语法题,
IS题
the difference between L and S is that the former is optimistic; the latter,pessimistic.
为什么不是the difference between L and S is that the former is optimistic,the latter,pessimistic.
2,IE题
some historians liken the reign of Russia's T,a contemporary of England's Q,famous for his brutal repression,to J's
J's为什么这个错了，还有为什么Russia's没错，特殊用法吗？不是应该用Russian吗？
IS题
in years past,teenagers typically passed notes to their friends in class;today they send electronic instand messages.
为什么不是：in years past,teenagers typically passed notes to their friends in class rather than electronic instand messages today.
4,IE题
in the aftermath of the space shuttle explosion,where seven crew members were killed``````
答案是where改成In which，但where不就等于in which吗？为什么一定还要换啊？