对比的两个对象可以是不同的事物，也可能是同一事物的不同时期.如果A和B进行对比，A的反面就是B。通常来说，各个参数完全相反。这个原理，可以运用到
What can be inferred from be fact that Lechuguilla Cave is no longer forming?
A. The cave has stopped attracting visitors
B. The cave no longer contains any limestone
C. The air in the cave is safer to breathe now than it was in the past
D. Gypsum deposits inside the cave are growing thicker
本题有一定的难度，我们来看看听力文本。
And Lechuguilla is pretty much dormant now. It’s not really forming any more. But, there is other ones like it, for example, in Mexico, that are forming. And when cave researchers go to explore them, they see and smell, the sulfuric acid and gases of...er...phew...now, something else, think of rotten eggs. And, it’s not just the smell. Explorers even need to wear special masks to protect themselves from the gases in these caves.
翻译：龙舌兰洞现在已经几乎休眠了。它现在几乎不再形成。但是，有其他一些像龙舌兰洞一样的岩洞，比如说在墨西哥的一些，还正在形成。当岩洞研究者去探索这些岩洞时，他们会看见，闻到硫酸等气体···额，噗，简直要人命，想想臭鸡蛋的味道。而且，不仅仅气味难闻，探索人员甚至需要戴特质面具，保护自己不受到这些岩洞中的气体的伤害。
解析：这段信息提到L-cave和与之类似的墨西哥cave。But转折说明两个岩洞进行对比，因此笔记应该分成上下两排：
L-cave: ×form ___
M-cave: √form danger/harm
由于两个岩洞进行对比，故M-cave的反面即L-cave的情况，由此可以推出L-cave现在不那么危险。