[听力原文] 19Due to the large number of tall buildings lining narrow streets, many big-city dwellers spend their days mostly in shadows. Cairo, for example, is one of the world’s densest large cities, with so many alleyways that light can barely reach, even when the sun is directly overhead. That’s why scientists there have developed a way to harvest the sun’s rays and redirect them into Cairo’s many tight streets and alleyways to bring light to thousands who otherwise would be in the dark.
Simple reflective surfaces like glass mirrors can shoot light in only one direction. Amr Safwat and his colleagues at Ain Shams University in Cairo have developed unique panels 20with a precise, computer-cut designed to catch the sun’s rays at any angle. At any time of day, any day of the year, the panels can gather up sunlight and redirect it to a predetermined point; they don’t need to move with the sun or be repositioned for different times of the year. 21Prototype testing showed that the cheap and easy-to-produce panels could potentially increase the amount of natural sunlight in any given alleyway by 200 percent in autumn and 400 percent in winter. With the same material used to make glasses, the panels are cheap and easy to produce—each one-square-meter panel and frame will cost between $70 and $100, and Safwat estimates it will take four one-meter panels to keep one block’s worth of alleyway lit.
Why do many big-city dwellers spend their days mostly in shadows（）
A. Because the city is full of people. B. Because of the weather pollution. C. Because the dwellers don’t like sunshine. D. Because of the tall buildings lining narrow streets.