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

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→)A. oneB. thatC. itD. this解析：考查代词。one表泛指，在这里作a strange disease的同位语。

这个问题我还想问问老师呢