----when has he started boxing?---- I suppose.A. InB. SinceC. BeforeD. After

问题补充：

----when has he started boxing?

----, I suppose.

A. In

B. Since

C. Before

D. After

答案：

A. In

(答案→)B. Since

C. Before

D. After

解析：主句是现在完成形式，所以选Since。 Since he has started boxing.

的状语（副词，介宾形式或主语从句）后面跟上一个一般疑问句形式。