题目

下列分子式表示一定是纯净物的是       A．C5H10                     B．C7H8O                   C．CH4O                D．C2H4Cl2  答案：C解析:  --Why does the lake smell terrible?--Because large quantities of water _______.A. have polluted B. is being pollutedC. has been polluted D. have been polluted