问题补充：
—I phoned you at 9 this morning but got no answer.
—I am sorry. I the bell. I in the garden.
A. didn’t have heard, must have been
B. didn’t have heard, must be
C. didn’t hear, must have been
D. didn’t hear, must be
答案：
A. didn’t have heard, must have been
B. didn’t have heard, must be
(答案→)C. didn’t hear, must have been
D. didn’t hear, must be
解析：情景对话句意为：--今天早上九点我给打了个电话不过没人答复
--很抱歉。没有听见电话声。我那时一定在花园吧
一般过去时：简单陈述一件过去发生的事，强调事件本身。Must have done 是must对过去事件的肯定推测，这个事件肯定发生了，所以用完成时。结合语境可知，选C.
