PT 61 S4 Q20

Many important types of medicine have been developedfrom substances discovered ... ...

Which one of the following is an assumption required by the argument?
(A) There are substances of medicinal value contained in tropical rain forest plants not yet studied by scientists that differ from those substances already discovered in tropical rain forest plants.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Complete Section: LR-B, Oct. '10 LSAT (PT 61 Sec. 4)