Only then ___how much damage had been caused.A has she realized B did she realized为什么不选A呢?

Only then ___how much damage had been caused.then当时就在当时他才意识到引起了多少的损失.当时：一般过去时态呢如：Only then did it get on to your computer.就在那时你才能从你的电脑上看到它.所以,不选A现在...能不能理解为该句的谓语是过去某时的动作，需用一般过去时，还是说以后遇到then就是用过去时态呢then有很多意思，不一定总是用过去时态的。这是将来的thenOnly then will Americans be safe.只有那时，美国才能是安全的。