设有复数类COMPLEX，在复数类中重载乘法运算符。 下列哪项是运算符重载的正确的声明格式？

A、COMPLEX *(COMPLEX c1, COMPLEX c2);
B、COMPLEX * (COMPLEX c2);
C、COMPLEX operator * (COMPLEX c1, COMPLEX c2);
D、COMPLEX operator * (COMPLEX c2);