单选题 The sheer diversity of tropical plants 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] 各选项的意思是：exploited利用的；controversial有争议的；inexhaustible无穷无尽的，用不完的；remarkable引人注目的，不寻常的。根据句意可知，答案是C。