In addition to the farming they since the 1990s the Smiths have started a small business

问题补充：

In addition to the farming they since the 1990s, the Smiths have started a small business.

A. have done

B. had been done

C. were doing

D. have been doing

答案：

A. have done

B. had been done

C. were doing

(答案→)D. have been doing

解析：考查时态。从时间状语since the 1990s，可知应该使用现在完成时或现在完成进行时，然后根据后面主句的谓语动词have started可推断，do 这个动词还在继续，故使用现在完成进行时。句意：自从20世纪90年以来，史密斯夫妇开了一家小公司，此外他们还一直在做农活。