If you are writing or studying , it makes very much difference where the light comes from. People use books and pens every day have to be especially careful about the way the light shines on their work. Every house gets its light either from daylight through the windows-which is the best to use-or from lamps or electricity；but whichever kind of light it is, the way it shines toward our book or work is a matter of great importance to the eyes. Take a book , sit with your back toward the window and to read. Your shadow( 影子) falls all over the page and makes it almost as bad for your eyes if you were in a dark room. Now tum around and face the window. page is in the shadow again , while the bright light in your eyes. Try sitting with your right side toward the window. This is very well for reading , but you were writing ,the shadow of your hand would fall across the and bother(打搅)you a little. There is just one other way: sit 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 to sit is always the same. 49. When you sit with your face towards the window ,（ ）·
A.your shadow falls on your book B.your book is in a shadow C.the light is still dark D.the light is on your page