He when the UFO arrived. He didn’t wake until the UFO disappeared.
A. slept
B. was sleeping
C. was doing homework
D. was singing
网友回答
A. slept
(答案→)B. was sleeping
C. was doing homework
D. was singing
解析：由下文句意“当UFO消失时他才醒来”可知前句意为“当UFO到达时他正在睡觉”。过去某一时刻正在做某事，用过去时行时。故选B。