My brother has been teaching he was 21 years old.A. whenB. ifC. forD. since

问题补充：

My brother has been teaching he was 21 years old.

A. when

B. if

C. for

D. since

答案：

A. when

B. if

C. for

(答案→)D. since

解析：主句用的是现在完成时态。since，自从……（到现在）现在完成时的标志词。句意：我的弟弟自从21岁就一直在教学。