My book is gone.Who---have taken it?

My book is gone.Who---have taken it?
为什么要选could
What struck the audience most was__the blind girl could accomplish with her won hands.
答案是what 是不是因为 accomplish是及物动词,what 做宾语

因为问者本人也不能确定是别人拿走了他的书还是自己弄丢了,这里只表示这种可能性,即“可能是谁拿走了呢”.情态动词里表示可能性的只有could,may,might,其中用could比较委婉,这里应该用could.
至于第二个你想的是对的.