Explanation
A. The passage does not provide specific data on the proportion of rain-forest plant species with disease-fighting properties. Therefore, we cannot infer that only a tiny proportion have these properties.
B. The passage does not address the geographical effectiveness of potential medicines derived from rain-forest plants. There is no mention of diseases that are specifically endemic to tropical regions.
C. (Correct Response) The passage, particularly in the third paragraph, indicates that many scientists believe undiscovered rain-forest plants may contain agents with unique disease-fighting properties and that these plants can offer crucial clues for medical research. This suggests that the value of rain-forest plants is, in part, from their potential contribution to medical research.
D. The discussion in the first paragraph regarding the absorption of carbon dioxide and release of oxygen by rain-forest plants relates to environmental functions, not to their medicinal properties.
E. The passage does not suggest that rain-forest plants with disease-fighting properties are nearly extinct. Instead, it suggests that there are many species yet to be discovered that may have such properties.
In the passage's third paragraph, it is implied that the scientific value of rain-forest plants is associated with their potential undiscovered medicinal properties, which are important for medical research. This part of the passage supports the inference that the value of these plants is partly derived from their role in providing materials for medical research.