请问补充250题128题

128. During the Renaissance, scholars were uncertain as to the location of Troy, and by the eighteenth century many historians doubted that Troy had ever existed.

(A) doubted that Troy had ever existed

(B) doubt that Troy has ever existed

(C) were in doubt as to the existence of Troy

(D) were doubtful concerning Troy’s existence

(E) had doubts about Troy’s ever existing

Key is A.  But why D is not correct?

仿佛D更能体现一种平行关系：were doubtful 和were uncertain相对应，concerning和as to 相对应。

这是平行的陷阱

A的意思表达更清晰明了

doubtful about是固定用法 没有doubful concerning

很清楚。谢谢！