18. A biologist does not merely describe organisms, but tries to learn ______ act as they do.
(A) what causes them to
(B) causes them to what
(C) what causes the
(D) which causes the

這是一題典型的 間接問句 + 名詞子句結構 的考題。

✅ 原句為：

A biologist does not merely describe organisms, but tries to learn ______ act as they do.

句意：
「生物學家不僅僅描述生物，而是試圖了解為什麼牠們會那樣行動。」

? 分析重點：

這裡的空格需要一個完整的 名詞子句，做為動詞 learn 的受詞，來說明 想了解的內容是什麼。

✅ 選項分析：

(A) what causes them to ✅

✔ 正確！
這是標準的 名詞子句結構：

• what causes them to act as they do = 是什麼導致牠們這樣行動

• 完整句：tries to learn what causes them to act as they do ✔

(B) causes them to what ❌

✘ 語序錯誤，"what" 不應在句尾出現，語法不通。

(C) what causes the ❌

✘ 不完整，後面缺少名詞受詞，例如 "what causes the behavior"
但原句後面直接接 "act as they do"，這樣搭不起來。

(D) which causes the ❌

✘ "which" 不是問句的開頭詞（what才是），而且整體結構不構成名詞子句。

✅ 正確答案是：

(A) what causes them to

補充句型知識：

常見句型：「learn / understand / find out + what/why/how + 主詞 + 動詞」

• I want to learn why birds migrate.

• Scientists study how genes affect behavior.

• He found out what caused the explosion.