GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 759_A7AN5QG8742AJ

According to the passage, solar-energy systems located on Earth must store energy for which of the following reasons? A. At any given location on Earth, the amount of sunlight available frequently changes. B. The operation of such systems depends on devices that require a power source other than solar energy C. Demand for the energy produced by such systems varies in unpredictable ways