—Why didn't you answer my phone when I phoned you at about 11 p. m. yesterday?
—I and failed to hear the phone.
A. might have slept
B. must be sleeping
C. might be sleeping
D. must have been sleeping
网友回答
A. might have slept
B. must be sleeping
C. might be sleeping
(答案→)D. must have been sleeping
解析：本题可以采用排除法。问句中提供整个过程表示过去的行为。情态动词后接完成时表示对过去的猜测，故排除B、C。问句中有“at about 11 p．m．”具体的时间点，所以选D。