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？

我的理解啊,不一定对.
1、我觉得是省略吧.你知道两个句子同一谓语是可以省略的吧,所以后半句是一个完整句子,the latter is pessimistic.
2、liken to 是搭配,把……比作,前后应该一样.应该是the reign of J.
加's没错啊,就是“的”的含义吧.
3、instant吧……?rather than前后两个结构必须一样.而前面的结构是passed notes to their friends,如果后面不加动词,意味着和notes用同一谓语.但是electronic instant messages应该用sent,不是passed...故上面的对.
4、只有引导定语从句,在定语从句中作地点状语,定语从句与先行词的搭配缺少介词 in 时,既能用 where,又能用 in which.其实这个我也不甚明白,只知道并不是所有的情况in which都能等同于where的