【题目】--- I called you last night, but no one answered.
--- Sorry, I _______ sleeping.
A.must have been
B.would have been
C.must be
D.could be
在线课程【答案】A
【解析】
考查情态动词和时态。句意：——昨晚我给你打电话了，但没人接。——对不起，我一定是睡着了。分析可知，前文提及打电话无人应答，后文解释原因：肯定当时在睡觉。must have been doing表示肯定当时正在做某事，符合语境要求。C项must be doing表示肯定现在正在做某事，与语境不符。故选A项。