Animals were able to move and intermix with 26.
答案：(B) one another
one another 是相互代名詞，意思是「彼此」，本身就是完整的受詞，指所有動物之間互相往來混居；another 後面須接單數名詞或指「再一個」，other 是形容詞須接名詞，the other 指兩者中的另一個，三者都不合。
another、other、the other、one another 四個字長得像，你背下來的大概是「another + 單數名詞、other + 複數名詞、the other ＝ 兩者中的另一個」這一組——規則本身沒錯，問題是它不完整：這一套原本有四個成員，你只記了三個。第四個 one another 不是「one + another」，它是獨立的代名詞，意思是「彼此、互相」，本身就是完整的受詞，後面不必再接名詞。真正的難處在中英差異：中文的「互相」是副詞放在動詞前，英文的 one another 是代名詞放在受詞位置。照中文語序去找副詞，就會漏掉它。
劍橋《English Grammar Today》把 each other 與 one another 並列為相互代名詞（reciprocal pronouns），用於主詞的成員彼此對對方做某件事。本句主詞 Animals 是複數，intermix（混居、往來）本身就是相互性的動作，需要的正是相互代名詞。整段講的是盤古大陸上眾多動物往來混居，不是兩隻動物互看。
another ＝ 再一個，後面接單數名詞或單獨當代名詞（another cup）；other 是形容詞，後面一定要接名詞（other people）；the other ＝ 兩者之中的另一個（one hand… the other hand）。三個都填不進一個需要「彼此」的位置。這一組一句記完：Lonely people need to reach out to one another, one small message after another, until each other's silence is broken.（寂寞的人需要彼此主動聯繫，一則短短的訊息接著一則，直到彼此的沉默被打破。）〔情境取材自 106 指考作文題：如何排解寂寞〕
interact／compete with one another（互相交流／競爭）、help each other（互相幫助）、one after another（一個接一個）、each other's + 名詞（彼此的…）、be separated from（與…分隔）、break up into（分裂成）。
Up to about 250 million years ago the world had just one huge super-continent called Pangaea. Animals were able to move and intermix with 26. About 200 million years ago this super-continent broke up into two continents: Laurasia and Gondwana. About 60 million years ago Gondwana broke up into 27 later became South America, Africa, Antarctica, India and Australia.
Since then Australia 28 from the rest of the world by vast oceans. The animals there no longer had 29 with animals from other parts of the world. They evolved separately. As a result, about 95 % of Australia's mammals and 94 % of its frogs are found 30 else in the world. Australia has lots of very unusual animals.