----You seem to have known Dr Smith for a long time.-That’s truehe joined our club

问题补充：

----You seem to have known Dr Smith for a long time.

---- That’s true. he joined our club six years ago.

A. Since

B. Until

C. When

D. Before

答案：

(答案→)A. Since

B. Until

C. When

D. Before

解析：介词辨析. A.自从…… B.直到，常和not一起连用；C.当……时候 D. 在……之前。根据题意，你好像认识史密斯博士好久了。——这是真的。自从他六年前加入我们俱乐部以来就认识。故选A。