两道语法题

25. Financier Andrew Mellon donated most of his magnificent art collection to the National Gallery of Art, where it is now locating.

collection为什么不是复数？当它意思是收藏时，它就是可数，当它泛指时才是不可数？？这里是泛指吗？怎样定义泛指呢？？

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15. A challenging new area in inorganic chemistry is ---- the role of transition metals in the biochemical catalysts called enzymes.

     (A) that of understanding

     (B) to have understanding

     (C) the understanding

     (D) understanding that

答案是A

collection 收藏品

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答案是D，collection没有错哦？