The reseach is so designed that once__nothing can be done to change it.A begun B begins C hzving begun D beginning为什么选A

问题描述:

The reseach is so designed that once__nothing can be done to change it.
A begun B begins C hzving begun D beginning
为什么选A

我觉得B也可以
如果虚拟的话为什么CAN 不是COULD?

假设开始,是虚拟的.