1.the letter ____ by him hasn't been posted yet
A written B was written (why A not B)
2.He was often seen___ with the peasants.
A working B to be worked(why A not B)
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?)
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)
5.He arrived early ____ tell us ____ go there.
A.in order to , don't B so as to, not to(答案是D 为什么B不行强调解释下第二个横线)