TPO29 閱讀第二題 的最後一題 - 英檢

請教TPO29 R2的最後一題
請問(C)選項為何錯誤?

原文中相關的句子是:
To what extent competition determines the composition of a community and the
density of particular species has been the source of considerable controversy.
The problem is that competition ordinarily cannot be observed directly but
must be inferred from the spread of increase of one species and the concurrent
reduction or disappearance of another species.


(C) Investigation of the ecological role of competition is difficult
because ordinary the competition cannot be observed directly and must be
inferred from its presumed effects.


看網路上的解釋是說:
後半句在原文中提到了，但是並沒有與前半句的這樣的因果關係，不選。

但是我覺得前半句也沒有錯誤呀?
想請教為什麼不能選呢?
謝謝~~

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