每题语法请详细说明 可能答案有错 请指出

1.the letter ____ by him hasn't been posted yet
A written B was written (why A not B)
此句谓语是“hasn't been posted”,“written by him”被动式做letter的后置定语,选B 句子中就存在两个谓语了.
2.He was often seen___ with the peasants.
A working B to be worked(why A not B)
doing与to do做动词不定式来讲,一般doing表状态也就是用在过去、现在的时态,而to do一般表示目的,用在将来的时态.而句中often一词是关键.
3.They are busy ____ for the final exam these day.
A preparing B with preparing(答案是A be busy with v+ing 不行吗?)
be busy doing 我们一般都是这么用,其他的不要记了,以防混淆
4.Soon we saw the thief ____ in the crowd.
A disappear B disappears C disappeard(答案是A 为什么 B不行)
一般的感官动词（see,hear,smell...）后直接用动词原型!
第五题请这位同学把选项发的全面一些,没有D选项.