Even 19 supplies are sufficient or plentiful, they are increasingly at risk due to various forms of pollution.
答案：(D) where
Even where 意為「即使在…的情況下」，where 引導的是抽象的情境子句，不必指實體地點：即使供水充足的地方，也日益受到污染威脅；how、why、what 都無法引導這種讓步子句。
答對率 26%。
where 你大概只用來指實體地點——公園、學校、城市，所以看到本句沒有具體地名就把它排除了。其實 where 可以引導抽象的「情境、狀況」：a situation where…（在…的情況下）、cases where…（在…的案例中）。本句 Even where supplies are sufficient ＝「即使在供水充足的地方（情況下）」。
句首的 Even 是關鍵——它需要一個讓步的框架，後面接的是完整子句 supplies are sufficient or plentiful，而這個子句不缺主詞也不缺受詞，所以要用引導副詞子句的 where，不是缺一角才用的 what。
前一句說淡水湖泊河流看似很多、其實只是杯水車薪；本句再退一步說即使充足的地方也有污染威脅——語氣是層層加重的，Even 正好承接。
Even where…（即使在…的情況下）、a situation／case where…（在…的情況／案例中）、This is where…（這就是…之處）。
More than 2.7 billion people will face severe shortages of fresh water by 2025. There is, 16, only a limited amount of water on the planet. Less than 3 percent of the world's water is fresh, and most of 17 is trapped in polar ice or buried underground in springs too deep to reach. Freshwater lakes, rivers and reservoirs may seem 18 but provide just a drop in the bucket.
Even 19 supplies are sufficient or plentiful, they are increasingly at risk due to various forms of pollution.
Fierce national competition 20 water resources has prompted fears that water issues contain the seeds of violent conflict. International cooperation is therefore by all means urgent.