the rat develop bacterial infection of the blood, as if their immune systems -the self-protecting mechanism against diseases-had crashed. 虚拟语气不是应该从句用过去分词,主句用would have done吗,为什么这句话用的是一般现在时
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If it is finetomorrow, I will go to visit my grandma.
If I were a bird, Iwould fly to America.
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