This missile is designed so that once _____ nothing can be done

This missile is designed so that once _____ nothing can be done to retrieve it.
A、fired
B、being fired
C、they fired
D、having fired
【正确答案】：A
【题目解析】：句意：设计这种导弹的目的在於一旦它发射了,任何举措都不可以重新找回它.  因为once 它后面是一种假设的情况呀,所以就得用fired,虚拟语气来的。这句话它省略了一些东西,完整的句子应该是这样的:this missile is desingned so that once this missile is fired nothing can be done to retrieve it.