the boy was walking down the street the UFO landed.A. WhileB. AfterC. BeforeD. until

问题补充：

the boy was walking down the street, the UFO landed.

A. While

B. After

C. Before

D. until

答案：

(答案→)A. While

B. After

C. Before

D. until

解析：句意：当这位男孩在街道上散步时，UFO着陆。While当……的时候；after在···……之后；before在……之前；until直到。所以选A。