虚拟语气if省略与不省略的区别if the policemen had arrived two minutes late

虚拟语气if省略与不省略的区别
if the policemen had arrived two minutes later than that,the man must have been killed by the gang.
a.had the policemen arrived
b.if the policemen arrived
c.if the policemen arrived
d.had the policemen had arrived
原句中的if the policemen had arrived被标为错误的,从下面四选项中选出正确的.答案选的是a.不理解啊不理解.觉得原句就没有错误啊 而且正确答案不就是把if给省略了么?