问题
选择题
My brother has been teaching ________ he was 21 years old.
A．when
B．if
C．for
D．since
答案
答案：D
题目分析：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。
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My brother has been teaching ________ he was 21 years old.
A．when
B．if
C．for
D．since
答案：D
题目分析：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。