例题：

Noisy as it was, he went on reading as if nothing _____.

A. had been happening

B. were happening

C. had happened

这道题答案给的是B，之前我在英语答疑网上也看到过对这道题的解释，是这么说的：

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链接：http://ask.yygrammar.com/app_q-27610.html 

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填 were happening 和 had happened 都是虚拟语气，但用法不同：用 were happening 表示当时正在发生的情况，用 had happened 表示针对当时来说属已经发生的情况。对于你的句子来说，很显然 were happening 更符合句子语境，指“吵闹”的情况当时“正在发生”。句意：

Noisy as it was, he went on reading as if nothing were happening. 虽然很吵，但他仍继续看书，好像什么事都没发生一样。（指当时本来很“吵闹”，但对他来说，好像当时“吵闹”的事没有正在发生）

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下面是我在网上查到的关于as if的资料：

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as if 从句用虚拟语气，当说话人认为句子所述的是不真实的或极少有可能发生或存在的情况时。从句虚拟语气动词时态的形式如下：

1、如果从句表示与现在事实相反，谓语动词用一般过去时。

2、从句表示与过去事实相反，谓语动词用“had＋过去分词”。

3、从句表示与将来事实相反，谓语动词用“would／could／might＋动词原形”。例如：

You look as if you didn’t care．你看上去好像并不在乎。

He talks about Rome as if he had been there before．他说起罗马来好像他以前去过罗马似的。

He opened his mouth as if he would say something．他张开嘴好像要说什么。

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这样的话，我们应该怎么解释were happening表示过去虚拟的用法呢？

还有，“He laughed as if he saw something funny"这个地方saw是表示过去虚拟的吗，应该怎么解释呢？