Friends are angels who lift us to our feet our wings have trouble remembering how to fly.

问题补充：

Friends are angels who lift us to our feet our wings have trouble remembering how to fly.

A. unless

B. because

C. though

D. when

答案：

A. unless

B. because

C. though

(答案→)D. when

解析：考查状语从句。句意：当我们遇见麻烦的时候，朋友就是帮助我们的天使。A除非；B因为；C尽管；而本句是指时间状语。故D正确。