I met a friend of mine I was walking in the park.A. beforeB. whenC. becauseD. until

问题补充：

I met a friend of mine I was walking in the park.

A. before

B. when

C. because

D. until

答案：

A. before

(答案→)B. when

C. because

D. until

解析：before在……之前；when当……时；because因为；until直到。句意：当我正在公园散步时，我遇到我的一个朋友。结合语境可知选B。