阅读下列短文，然后根据短文的内容从每小题的四个选择项中选出最佳的一项。

     If you are writing or studying, it makes very much difference where the light comes from. People who use books and pens every day have to be specially careful about the way the light shines on their work.
     Every house gets its light either from daylight through windows which is the very best to use or from the lamps or electricity; but whichever kind of light it is, the way it slants towards our books or our work is a matter of great importance to the eyes.
     Take a book, stand with your back toward the window, and try to read. Your shadow (影子) falls all over the page and makes it almost as bad for your eyes as if you were in a dark room.
     Now turn around and face the window. The page is in the shadow again, while the bright light is in your eyes.
     Try standing with your right side toward the window. This is very good for reading, but if you were writing, the shadow would fall across the page and bother (打搅) you a little.
     There is just one other way: stand with your left side to the window. Now everything is perfect for reading and for writing, too.
     Whatever kind of light is in the room, the rule about the right way to sit is always the same.