Freshwater lakes, rivers and reservoirs may seem 18 but provide just a drop in the bucket.
答案：(C) numerous
numerous 意為「數量繁多的」，與後半句 provide just a drop in the bucket（只是杯水車薪）形成 but 的對比：淡水湖泊河流看似很多，實際上遠遠不足；scarce（稀少的）會與 but 的轉折衝突。
答對率 34%。
看到後半句說「只是杯水車薪」，你大概會選 scarce（稀少的）——但這樣一來 but 就沒有東西可以轉折了：「看起來很少，但只是杯水車薪」前後同向，句子不成立。「看到 but，就要確定前後是相反的」；空格必須是正面、數量多的意思，才和後半形成落差。
may seem numerous but provide just a drop in the bucket ＝「看起來數量很多，實際上只是杯水車薪」。seem（看起來）本身就在暗示「與事實有出入」，是這類對比句最常見的訊號詞。
字面是「水桶裡的一滴水」，意思是「微不足道、杯水車薪」，與中文的「九牛一毛」同義，用在數量遠遠不足的情境。
numerous（數量繁多的）、seem…but…（看似…卻…）、a drop in the bucket（杯水車薪）、be at risk（面臨風險）。
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.