Rising temperatures, an effect of global warming, 25 threaten the ecosystem of this mountain area.
答案：(A) thus
前文已建立「升溫→融雪→缺水」的因果鏈，thus（因此）承接這個因果推論，表示氣溫上升「因此」威脅生態系統；just（只是）、instead（取而代之）、otherwise（否則）都不符合這個因果邏輯位置。
空格夾在逗號之後、動詞 threaten 之前，你會覺得「這個位置該放修飾動詞的副詞吧，thus 這種銜接用的詞怎麼會插在句子中間」，於是改選看起來比較像一般副詞的 just。答對率只有 49%，位置造成的誤判是主因之一。
第一步先把句子主幹抓出來。Rising temperatures, an effect of global warming, ______ threaten the ecosystem...，中間那段 an effect of global warming 是同位語，用兩個逗號夾住，拿掉完全不影響句構。主詞是 Rising temperatures（複數），動詞是原形的 threaten，空格正好落在主詞與動詞之間。thus、therefore、however 這一類詞歸在「連接副詞」（linking adjunct）底下，可以出現在副詞能出現的任何位置：句首、主詞與動詞之間、助動詞之後，甚至句尾（劍橋 English Grammar Today）。所以 thus 放在主詞與動詞中間是完全正常的位置，不是印錯也不是特例。這一點記起來，往後在句子中間看到 however、therefore 就不會慌。第二步看邏輯關係。前面兩句已經鋪好一條因果鏈：積雪消失，於是沒有足夠的水分滋養動植物。本句的主詞 Rising temperatures 正是造成融雪的源頭，thus（因此）把「暖化升溫」與「威脅生態」串成結論，銜接得上。
just（只是、僅僅）：會把語氣削弱成「只是威脅一下而已」，與整段強調事態嚴重的語氣相反。instead（取而代之、反而）：要先有一個被取代的對象或被推翻的做法，前文沒有。otherwise（否則、不然）：要先有一個假設或條件才談得上「否則」，前一句是陳述事實。
and thus（因而）、as a result（因此、結果）、an effect of（…所造成的影響）、have an effect on（對…有影響）、result in（導致…）、result from（起因於…）。
Every year tens of thousands of tourists visit Mount Kilimanjaro, the highest mountain in Tanzania, Africa, to witness the scenes depicted in Earnest Hemingway's The Snows of Kilimanjaro. They are attracted by the American writer's 21 of the millennia-old glaciers.
However, this tourist attraction will soon 22 . According to the Climate Change Group, formed by environmentalists worldwide to document the effects of global warming, Mount Kilimanjaro's snows and glaciers are melting and are 23 to disappear by 2020.
Not only will the summit lose its tourist attraction, but the disappearance of the snows will also cause major damage to the ecosystem on the dry African plains at its base. 24 the snow covering the peak, there will not be enough moisture and water to nourish the plants and animals below. Rising temperatures, an effect of global warming, 25 threaten the ecosystem of this mountain area. The loss of snows on the 5,892m peak, which have been there for about 11,700 years, could have disastrous effects on Tanzania.