people were watching the sky the alien got out of the UFO.A. IfB. WhenC. BecauseD. While

问题补充：

people were watching the sky, the alien got out of the UFO.

A. If

B. When

C. Because

D. While

答案：

A. If

B. When

C. Because

(答案→)D. While

解析：while引导的时间状语从句常用进行时态.