Then, almost 26 at a given signal, the group begins to march toward the colony 27.
答案：(C) to be raided
不定詞表目的（被動形式 to be raided），「朝著即將被襲擊的聚落前進」，表示行軍的目的是去襲擊該聚落；raided（過去式/過去分詞，不能這樣單獨接在colony後面表目的）、raiding（現在分詞，主動語意方向不合，應該是聚落「被」襲擊）、to be raiding（不是標準用法）都不合。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個選項是 raid 的四種形態，而過去分詞 raided 表被動，看起來最合「被襲擊的蟻穴」這個意思。但這時候襲擊還沒開始——時間點不對。
先看結構：the group begins to march toward the colony ___（這群螞蟻開始朝那個……的蟻穴前進）。空格帶出的一串是用來修飾 colony 的。先分主動被動：蟻穴是被襲擊的一方，所以要被動，raiding 與 to be raiding 這兩個主動形式先出局。剩下 raided 與 to be raided，關鍵在時間點：過去分詞 raided 表示動作已經完成——「已經被襲擊過的蟻穴」；但這一句是螞蟻剛出發、還在行軍途中，襲擊還沒發生。不定詞的被動 to be raided 表示「即將被、預定要被」，正好符合「牠們正朝著那個即將被襲擊的蟻穴前進」。不定詞當形容詞修飾名詞、表示未來或應該做的事，是很常見的用法：the work to be done（待完成的工作）、the house to be sold（待售的房子）、the next train to arrive（下一班即將進站的列車）。再往後讀一句驗算：When the Amazons reach the nest of their intended victims, a fierce battle may take place.（當亞馬遜蟻抵達預定目標的巢穴時，一場激戰可能就此展開。）——intended（預定的）與 may take place（可能發生）都指向未來，證實襲擊確實還沒開始。
raided：表已經完成，時間點不對。raiding、to be raiding：兩個都是主動，方向錯了——蟻穴不會去襲擊別人。
the colony to be raided（即將被襲擊的蟻穴）、the work to be done（待完成的工作）、raid a nest（突襲巢穴）、intended victims（預定的目標）、a fierce battle（激戰）、take place（發生）。
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.