此關係代名詞不易懂 - 考試

91高檢的英文題目，有句子如下：

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Wool is the most obvious characteristic on which to classify breeds of sheep.

羊毛是用以分級綿羊種類最明顯的特徵.

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1.請問on which 指的是甚麼?

2.他是代表 主格/受格/所有格 哪一種呢?

3.還有, on which to classify breeds of sheep 看起來不是一個完整的句子,
關代與後面子句,不是應成為一完整的句子嗎?有上述題目這種用法嗎?

感謝英文高手解惑，謝謝。

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