Pollutants in the atmosphere can cause acid rain. While acid rain in itself cannot significantly affect the acidity of bodies of water into which it falls, it can greatly increase the acidity of nearby lakes by increasing the amount of decaying matter on a forest floor. A recent increase in the acidity of the water in Forest Lake, therefore, surely indicates that the rain falling nearby has become more acid.

Which of the following, if true, most seriously weakens the argument?

  答案为Large-scale logging, which was recently begun in the forest surrounding Forest Lake, has increased the amount of decaying matter on the forest floor.

它还有一个选项为Even in areas without significantly amounts of acid rain, most lakes in regions with vegetation similar to the vegetation around Forest Lake have acidity levels higher than those of other lakes.为什么不可以呢？

原文本身推理本身存在问题，即A-〉B，不能得到B-〉A

答案为标准的他因weaken;

另外那个选项应为无关选项，注意原文中的increase，它只说明与FL植被

相似的湖的酸度一般比较大，但没有解释为什么近期增加了。