[問題]1998年8月 Q8 (structure)

[林威]

(8)
In this pure state antimony has no important uses, but ___ with other substances, it is an extremely useful metal.
(A) When combined physically or chemically
(B) combined when physically or chemically
(C) the physical and chemical combination
(D) it is combined physically and chemically


答案是A
我選的是C
麻煩高手解答一下 謝謝

[micki]

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[sickkid]

___ with other substances, it is an extremely useful metal.

but 後接一個完整的句子（從句，主句）
原為When it was combined physically or chemically, it is an extremely useful metal.

子句與主句，主詞相同，所以，子句的主詞可省，被動改過去分詞變成
When combined with other substances physically or chemically, it is an extremely useful metal.

甚至when也可以不要了，依題意還是留著when句意比較清楚些