Critical Reasoning

Naturally occurring chemicals cannot be newly patented once their structures have been published. Before a naturally occurring chemical compound can be used as a drug, however, it must be put through the same rigorous testing program as any synthetic compound, culminating in a published report detailing the chemical’s structure and observed effects.
If the statements above are true, which of the following must also be true on the basis of them?
(A) Any naturally occurring chemical can be reproduced synthetically once its structure is known.
(B) Synthetically produced chemical compounds cannot be patented unless their chemical structures are made public.
(C) If proven no less effective, naturally occurring chemicals are to be preferred to synthetic compounds for use in drugs.
(D) Once a naturally occurring compound has been approved for use as a drug, it can no longer be newly patented.
(E) A naturally occurring chemical cannot be patented unless its effectiveness as a drug has been rigorously established.

Answer is D

I too bet on D
Whats the OA?

D seems to be the answer. What's OA?