Many important types of medicine have been developed from substances discovered in plants that grow only in tropical rain forests. There are thousands of plant species in these rain forests that have not yet been studied by scientists, and it is very likely that many such plants also contain substances of medicinal value. Thus, if the tropical rain forests are not preserved, important types of medicine will never be developed. Which one of the following is an assumption required by the argument? ['Most of the tropical rain forest plants that contain substances of medicinal value can also be found growing in other types of environment.', 'The tropical rain forests should be preserved to make it possible for important medicines to be developed from plant species that have not yet been studied by scientists.', 'Any substance of medicinal value contained in plant species indigenous to tropical rain forests will eventually be discovered if those species are studied by scientists.', '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.']