--- Why! I ________ get you on the phone.--- We ________ basketball on the playground when you phoned.A.didn't; must be playingB.couldn't; must be playingC.couldn't; must have playedD.didn't; must have been playing
网友回答
D解析第一句表述过去的真实情况，故用一般过去时；第二句表示就过去某时正发生的动作进行肯定推测，故用must have been doing。