The Amazons are completely dependent on their slaves 29 their sharp jaws, so well suited for fighting, are useless in taking care of larvae or for digging.
答案：(D) because
因果關係：「亞馬遜蟻完全依賴牠們的奴隸，因為牠們銳利的顎不適合照顧幼蟲」，because（因為）符合；unless（除非）、that、if（如果）都不合這個因果邏輯。
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四個選項裡三個都能帶出子句，形態上刷不掉，只能靠前後兩句的邏輯關係。
先把整句讀完：The Amazons are completely dependent on their slaves ___ their sharp jaws, so well suited for fighting, are useless in taking care of larvae or for digging.（亞馬遜蟻完全依賴牠們的奴隸……牠們那對非常適合戰鬥的銳利大顎，對照顧幼蟲或挖掘完全派不上用場。）前半是結果（完全依賴奴隸），後半是原因（自己的顎只能打架、幹不了別的活），這是順向的因果，填 because。注意那個插入語 so well suited for fighting（如此適合戰鬥）——它夾在主詞 their sharp jaws 與動詞 are 之間，而且與後面的 are useless 形成強烈對比：越是為戰鬥而生，就越不適合做別的事。讀長句時遇到兩個逗號夾住的成分，先用括號把它遮起來，主幹就清楚了。
unless（除非）：帶出排除性的條件，變成「除非牠們的顎沒用，否則就依賴奴隸」，邏輯繞不回來。if（如果）：帶出假設，但「顎不適合照顧幼蟲」是既成的事實，不是假設。that：可以帶出名詞子句，但 be dependent on 後面已經有受詞 their slaves 了，沒有位置再放一個名詞子句。
be dependent on（依賴）、be suited for（適合於）、be useless in V-ing（在……方面派不上用場）、take care of（照顧）、larvae（幼蟲，larva 的複數）、sharp jaws（銳利的大顎）。
Some species of ants keep slaves. Probably the best known for the practice is the large brownish-red Amazon ant. A slave raid starts with an assembly of the Amazon warriors outside their own nest. Then, almost 26 at a given signal, the group begins to march toward the colony 27.
When the Amazons reach the nest of their intended victims, a fierce battle may take place. The Amazons fight on 28 they seize the pupae of the raided nest. They carry these back home. In time the pupae develop into adults and spend their lives working for the Amazons.
The Amazons are completely dependent on their slaves 29 their sharp jaws, so well suited for fighting, are useless in taking care of larvae or for digging. Also, the masters become so lazy that, after a while, they lose their muscles, and they become too weak 30 to feed themselves. Finally, they become so unhealthy that they die.