Sunlight is free, but that is no reason to waste it. Yet even the best silicon solar cells—by far the most【C1】______sort—convert only a quarter of the light that falls on them. Silicon has the【C2】______of being cheap: manufacturing improvements have brought its price to a point where it is snapping at the heels of fossil fuels.【C3】______many scientists would like to replace it【C4】______something fundamentally better. John Rogers, of the University of Illinois, Urbana-Champaign, is one. The cells he has【C5】______can convert 42.5% of sunlight.【C6】______improved, Dr Rogers reckons, their efficiency could rise to 50%. Their【C7】______is that they are actually not one cell, but four, stacked one on top of another. Solar cells are made of semiconductors, and every type of semiconductor has a【C8】______called a band gap that is different from that of other semiconductors. The band gap【C9】______the longest wavelength of light a semiconductor can absorb (it is transparent to longer wavelengths). It also fixes the【C10】______amount of energy that can be【C11】______from shorter wavelength. The result is that long-wavelength photons are lost and short-wave ones incompletely utilised. Dr Rogers【C12】______this by using a different material for each layer of the stack. He chooses his materials【C13】______the bottom of the band gap of the top layer matches the top of the band gap of the one underneath, and so on【C14】______the stack. Each layer thus【C15】______off part of the spectrum, converts it efficiently into electrical energy and passes the rest on. The problem is that the materials needed to make these semiconductors are【C16】______But Dr Rogers has found a way to overcome this.【C17】______solar-cell modules are completely covered by semiconductor, but in his only 0.1% of the surface is so covered. The semiconducting stacks, each half a millimeter square, are【C18】______over that surface many dots. Each stack then has a pair of cheap glass lenses【C19】______over it. These focus the sun's light onto the stack, meaning that all【C20】______light meets a semiconductor.

【正确答案】 D

【答案解析】解析：Dr Rogers所研发的电池要克服传统电池不能有效利用光能的缺点，空格所在句提到“每一层……光谱的一部分，将其有效地转化为电能，并把剩下的光谱传递下去”。由此推出，电池的每一层要“截取”光谱的一部分，且填入的动词要能与off连用，所以D项chops“砍”为答案．chop off意为“砍掉”。