It used to be thought the Earth was flat.A. asB. whenC. sinceD. that

问题补充：

It used to be thought the Earth was flat.

A. as

B. when

C. since

D. that

答案：

A. as

B. when

C. since

(答案→)D. that

解析：it 为形式主语，此句为 People used to think that the Earth was flat 的被动语态形式。