请教一下同位语和which 的区别

zxq1989

In laboratory rats, a low dose of aspirin usually suffices to block production of thromboxane, (划线开始）which is a substance that promotes blood clotting, but not seriously interfering(结束）with the production of prostacyclin, which pervents clotting.
这题答案：a substance that promotes blood clothing, but does not seriously interfere
纠结选项：which is a substance that promotes blood clothing, but does not seriously interfere
嗯，我看PREP笔记时知道，GMAT倾向于选择同位语的句子，在这题中答案就是这样。但我发现这题没划线的后半句中，是用which修饰, 所以请问各位牛牛们，什么时候用WHICH, 什么时候用同位语，因为前阵子做过一道题貌似就是用同位语做迷惑选项，而正确答案是which,当时记得对于那道题的解释是用同位语可能会修饰主语造成歧义...我语法比较烂，还请各位指教，谢谢了!

还有一道题目，也请教下哈
One report concludes that many schools do not have, or likely to have, enough computers to use them effectively.
A or
D nor they are 正确答案

ludanthu

第一题，这个纠结的选项里which is就已经是定从了，进来a substance that又一个定从....明显的awkward and redundant

第二题先不论or 和 nor的意思不同，首先主句是do, 而likely应该和be配合，所以补出一个be是必然的