04-05-15， 04-10-15

04-05-15

15.Nylon was ___the human-made fibers.

A.      the first of which

B.      what the first of

C.      it the first of

D.     the first of

why the answer is D instead of B?

04-10-15

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

Why the answer is A although I chose it? who can explain the structure of this sentence?

Why not C? What about if we have another choice E. understianding?

Thanks

if choose c, it must be

the understanding of