请教一道语法题 关于that of

roundroundyy

请帮助我看看这道托福语法题，谢谢,请版主不要删，因为已经没有人在托福班面讨论语法了，谢谢
15. A challenging new areain inorganic chemistry is ---- the role of transition metals in the biochemicalcatalysts called enzymes.
(A) that of understanding
(B) to have understanding
(C) the understanding
(D) understanding that

选A 请问that of 在这里是什么成分
c为什么不对呢？还有把选项变成 to understand or understanding对吗？谢谢

sdcar2010

that of = the area of

还有把选项变成 to understand or understanding对吗？  No.

. . . is how to understand . . .