Many people have been ill with a strange disease these days we’d never heard of before.A

问题补充：

Many people have been ill with a strange disease these days, we’d never heard of before.

A. one

B. that

C. it

D. this

答案：

(答案→)A. one

B. that

C. it

D. this

解析：考查代词。one表泛指，在这里作a strange disease的同位语。