Because of the energy crisis, scientists in the oil-consuming nations have become increasingly interested in the potential (潜在的) solar energy. Some experts estimate that the present supply of fossil (化石) fuel (燃料) will not last until the end of the twentieth century. The problem that solar energy researchers face is how to use the sun’s energy effectively and inexpensively. One of the most popular methods currently being tested is to use rooftop solar collectors and underground storage tanks. An advantage of a properly working system of this type is that it will not create any environmental pollution. Another advantage of using solar energy is that the cost of the sun’s rays is zero. When a solar heating system is working at maximum efficiency, it can provide up to 80 percent of winter heating needs. The cost of using the sun’s rays for heating is ______.
A. about the same as the cost of fossil fuels B. several hundred dollars per year C. negligible compared with other energy sources D. determined by the severity of the winter