问题
选择题
“Show me your permit, please.” “Oh, it’s not in my pocket. It ______.”
A．might fall out
B．could fall out
C．should have fallen out
D．must have fallen out
答案
答案：D
既然“通行证现在不在口袋里”，那么它“掉出去”应发生在过去，故排除A和B。比较选项C和D所表示的意思，选D更恰当。