单选题 The diversity of tropical plants in the region represents a seemingly ______ source of raw materials, of which only a few have been utilized.

【正确答案】 C

【答案解析】[解析] 从句意尤其是后半句，可知此题应选C(用不完的，用之不竭的。如：an inexhaustible supply of coal用之不竭的煤炭供应)。A(被开采的，被开发的)、B(争论的，争议的)和D(不平常的，非凡的，显著的)均错误。