Also, the masters become so lazy that, after a while, they lose their muscles, and they become too weak 30 to feed themselves.
答案：(C) even
【句型】too...even to... 加強語氣「甚至太……以致於無法……」，even 加強 too weak to feed themselves 的程度；only／also／neither 都不符合這裡加強語氣的用法。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個選項都是很短的副詞，而空格夾在 too weak 與 to feed themselves 之間，一時看不出這個位置要放什麼。其實它是用來把 too ... to 的語氣再推高一級的。
先看句型：they become too weak ___ to feed themselves（牠們變得太虛弱，……無法自己進食）。too ... to 本身已經是「太……以致於不能」，空格要填的是再加強一層的字。even（甚至）正是用來標示一個極端的例子：虛弱到連「自己吃東西」這種最基本的事都做不到。整句是「牠們變得太虛弱，甚至連自己進食都辦不到」。even 的功能一直都是把一個最不可能、最極端的情況拉出來，89 年第 23 題考的是它放在句首的用法（Even people who say they never dream show about 20 percent of REM sleep，連自稱從不做夢的人也一樣），本題則是放在句中加強 too ... to 的程度。位置不同，功能相同：都是「連這個都……」。再往後讀一句驗算：Finally, they become so unhealthy that they die.（最後牠們變得非常不健康，因而死去。）——程度一路往下走到死亡，even 的加強語氣完全合拍。
only（只、僅）：放在這裡會變成「太虛弱到只能不自己進食」，語意不通。also（也）：表累加，但這一句不是在加一項新的事實，是在加強程度。neither（兩者都不）：要有兩樣東西可以否定，而且它通常放在句首並造成倒裝，這裡都不成立。
too ... even to V（太……甚至連……都不能）、too ... to V（太……以致於不能）、so ... that ...（如此……以致於）、feed oneself（自己進食）、lose one’s muscles（肌肉退化）、become lazy（變得懶惰）。
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.