[单项选择]The need for solar electricity is clear. It is safe, ecologically sound, efficient, continuously available, and is has no moving parts. The basic problem with the use of solar photovoltaic devices is economics, but until recently very little progress has been made toward the development of low-cost photovoltaic devices. The larger part of research funding has been devoted to study of single-crystal silicon solar cells, despite the evidence, including that of the leading manufacturers of crystalline silicon, that the technique holds little promise. The reason for this pattern is understandable and historical. Crystalline silicon is the active element in the Very successful semiconductor industry, and virtually all of the solid state devices contain silicon transistors and diodes. Crystalline silicon, however, is particularly unsuitable to terrestrial solar cells. Crystalline silicon solar cells work well and are successfully used in the space program, where cost is not an issue. Wh A. Solar and extraterrestrial. B. Photovoltaic devices and solar cells. C. Crystalline silicon and amorphous materials. D. Amorphous materials and higher efficiencies.