《水力发电厂计算机监控系统设计规范》规定：水电厂计算机监控系统应采用分层分布式结构，不包含（）那一层。

A.电厂控制级；

B.调度控制级；

C.现地控制级。

It's difficult for me to decide which one to choose, because ______ of them are good. [ ]

A. neither

B. both

C. either

D. each