Which statement characterizes a wind power system? 

A)  It has a low net useful energy yield. 
B)  It requires a long time to construct. 
C)  It provides an unlimited source of energy at favourable sites. 
D)  It emits moderate amounts of air pollution.

Wind power systems provide an unlimited source of energy at favourable sites, making it a sustainable and renewable form of energy. While wind turbines do have construction and installation costs, they can start producing energy relatively quickly compared to other energy sources. Wind power systems do not emit air pollution, making them a cleaner alternative to fossil fuels. However, their net useful energy yield may not always be as high as other energy sources.