It is a good plan in theory but it to be seen whether it works in practice. A. keepsB.

问题补充：

It is a good plan in theory, but it to be seen whether it works in practice.

A. keeps

B. stays

C. stands

D. remains

答案：

A. keeps

B. stays

C. stands

(答案→)D. remains

解析：考查动词：A. keeps保持，B. stays停留，C. stands站立D. remains仍然，句意：这在理论上是个好计划，但是否能在实践中起作用还有待证实。选D。