--- 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
A
【详解】
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。
过去完成进行时：
过去完成进行时，就是相对过去的某个时刻来说已经对现在有直接影响并且还在进行的动作。放在间接引语或虚拟语气中时它的时态不能再向前推，向后推是现在完成进行时。过去完成进行时是由"had been + 现在分词"构成。
例如：She had been suffering from a bad cold when she took the exam.
她在考试之前一直患重感冒。
过去完成进行时构成：
过去完成进行时是由"hadbeen+现在分词"构成。
如：She had been suffering from a bad cold when she took the exam. 她在考试之前一直患重感。
Had they been expecting the news for some time? 他们期待这个消息有一段时间了吧？
过去完成进行时用法：
1、表示过去某一时间之前一直进行的动作。
过去完成进行时表示动作在过去某一时间之前开始，一直延续到这一过去时间。和过去完成时一样，过去完成进行时也必须以一过去时间为前提。过去完成进行时也是一个相对的时态，上下文中须有明示或暗示的作为参照的过去的时间。
如：I had been looking for it for days before I found it. 这东西我找了好多天才找着。
如：They had only been waiting for the bus a few minutes when it came. 他们只等了几分钟车就来了。
2、表示反复的动作。
如：He had been mentioning your name to me. 他过去多次向我提到过你的名字。
3、过去完成进行时还常用于间接引语中。
如：The doctor asked what he had been eating. 医生问他吃了什么。
I asked where they had been staying all those days. 我问他们那些天呆在哪儿。
4、过去完成进行时之后也可接具有"突然"之意的when分句。
如：I had only been reading a few minutes when he came in. 我刚看了几分钟他就进来了。
She'd only been reviewing her lessons for a short while when her little sister in terrupted her. 她温习功课才一会儿，她妹妹就打断她了。
过去完成进行时和过去完成时的比较：
如：She had cleaned the office, so it was very tidy. 她已经打扫过办公室了，所以很整洁。(强调结果)
She had been cleaning the office, so we had to wait outside. 她一直在打扫办公室，所以我们不得不在外面等着。(强调动作一直在进行)
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