Seeing the enemy,Gary was behaving as though nothing____.

1. 答案的确是A.
2. 因为as if/though引导的方式状语从句或表语从句的虚拟语气的时态使用规则并不是直接根据虚拟的动作所发生的时间来确定的,而是看从句动作和主句动作对比而确定的,即：.
1）从句动作如发生在主句动作之前,则从句采用过去完成时即had+动词过去分词
如：He talked with me as if he had known me before.
“know me”发生在“talk with me”之前
2）从句动作如果和主句动作(几乎)同时发生,则从句只需用采用一般过去时或过去进行时就可以了.
如：He stood there silently as if he was talking with somebody.
3) 从句动作如果发生在主句动作之前,即当时看来还尚未发生的话,则从句采用过去将来时：would+动词原型
如：He looked sad as if he would die soon.
3. 从本题句意上看,其实“nothing happen”和“was behaving”是表达同时存在的动作,所以只需要采用一般过去时就可以了.

A happened
B 里面说的是 had，过去式。句子整体是现在进行时。
seeing， behaving所以不能用had。

敌人正出现，不是完成。看看这么说：gary was behaving as though he didn't see. 而不是 he hadn't see 考虑一下吧。