a certain doubt among the workers as to whether he could finish the task in a short time．

问题补充：

a certain doubt among the workers as to whether he could finish the task in a short time．

A. It was

B. There had

C. There existed

D. It had

答案：

A. It was

B. There had

(答案→)C. There existed

D. It had

解析：There existed为there be句型的变体意为“有”。句意：关于他是否能在短时间内完成这项任务在工人中存在着某种怀疑。