A
是非限定性定语从句,排除BC
as指代前面整个句子
B
that前不能加介词,排除c
句子还原为he could see what was going on through which
what was going on 是名词性从句,做see的宾语
通过小洞, 所以which前要有through
B
all指代人时,谓语用复数形式
指代物时,用单数形式
过去完成时要有过去的时间做比较
题干中没有,所以用现在完成时
that can be eaten做定语修饰all
真正的谓语是has been ...