问题补充：
— Why! I get you on the phone.
— We basketball on the playground when you phoned.
A. didn ; must be playing
B. couldn ; must be playing
C. couldn ; must have played
D. didn ; must have been playing
答案：
A. didn ; must be playing
B. couldn ; must be playing
C. couldn ; must have played
(答案→)D. didn ; must have been playing
解析：解题的关键在第二空。mustbe 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。
