The earliest vertical windmills were used in Persia more than 2,000 years ago for the grin

ding of grain. Windmills were adopted for pumping water in North America by the middle of the nineteenth century. Their use declined drastically in the 1930's when inexpensive electricity reached the rural areas. A renewed interest in the use of wind power to generate electricity followed the energy crisis of the 1970's. A program of the United States Department of Energy encouraged the development of new machines, the construction of wind farms, and an evaluation of the economic effect of a large scale Use of wind power.

Public acceptance of wind energy conversion systems is an important consideration in planning for the widespread application of wind energy. Studies have shown that the environmental impact of such systems is relatively small compared with conventional electric power systems. Wind-powered systems do not require the flooding of large land areas or the alteration of the natural ecology, as dohydroelectric systems. Furthermore; they produce no waste products or thermal or chemical effluents, as fossil-fueled and nuclear fueled systems do.

Which of the following is NOT an advantage of wind energy？

A．Wind energy does not require the flooding of large land areas.

B．Wind energy produces no waste products.

C．The availability of wind energy is not dependent on the weather.

D．Wind energy does not produce thermal or chemical effluents.