问题
选择题
— Why! I _____ get you on the phone.
— We ______ basketball on the playground when you phoned.
A．didn't; must be playing
B．couldn't; must be playing
C．couldn't; must have played
D．didn't; must have been playing
答案
答案：D
解题的关键在第二空。must be doing表示对目前下在发生的情况的推测，而根据题干的时间提示… when you phoned，可知是谈虎色变论过去情况，因此排除A、B现两项。must have done 表示对过去已经发生的行为进行推测；must have been doing表示对过去正在进行的动作推测。句意：你打电话来的时候，我们一定在在操场打蓝球呢。由此判定答案为D。