—How long do you think it is he arrived here?　　　　—No more than half a year I believe.A.

问题补充：

—How long do you think it is he arrived here?

—No more than half a year, I believe.

A. when

B. that

C. before

D. since

答案：

A. when

B. that

C. before

(答案→)D. since

解析：与下文的答语相连，完整结构是“从他来之后有半年了”。