请教道语法题

leecruise

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

that这里指代什么呢？understanding后面怎么可以直接接the role of ...

pumpkin

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语法，请去TOEFL语法专区。

http://forum.chasedream.com/index.asp?boardid=58

另外，发贴提问，请给出题目出处，看看语法区置顶的发贴说明。

方便自己方便大家。谢谢

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