A Botanist Was Visiting a Tropical Region for the Purpose

A botanist was visiting a tropical region for the purpose of discovering plants with medicinal properties. All of the following might be ways of identifying potentially useful plants except

A) observing which plants sick animals seek out.
B) observing which plants are the most used food plants.
C) observing which plants animals do not eat.
D) collecting plants and subjecting them to chemical analysis.
E) asking local people which plants they use as medicine.