It remains to be seen this plan is practical enough to keep the project going.A. whether

问题补充：

It remains to be seen this plan is practical enough to keep the project going.

A. whether

B. how

C. where

D. when

答案：

(答案→)A. whether

B. how

C. where

D. when

解析：A 考察名词性从句。句意：这个计划是否足够可行让这个工程继续下去还有待于进一步观察。What引导主语从句，it是形式主语，根据句中的remains to be seen可知此处是指一种不确定性。故A正确。