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

試題分析：主句用的是現(xiàn)在完成時(shí)態(tài)。since，自從……（到現(xiàn)在）現(xiàn)在完成時(shí)的標(biāo)志詞。句意：我的弟弟自從21歲就一直在教學(xué)。
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