The project is designed in this way and once____,nothing can be done to change it.A.startB.startingC.startedD.having started为什么是C? once started是固定用法吗?

The project is designed in this way and once____,nothing can be done to change it.A.startB.startingC.startedD.having started为什么是C? once started是固定用法吗?
The project is designed in this way and once____,nothing can be done to change it.
A.start
B.starting
C.started
D.having started
为什么是C? once started是固定用法吗?

The project is designed in this way and once____,nothing can be done to change it.A.startB.startingC.startedD.having started为什么是C? once started是固定用法吗?
once是一旦
而且是省略句!once it is started!由于主语相同,所以省略主语和谓语!

不是固定用法，句子是说一旦被开始，词形和designed一致，用started。