If you are writing or studying, it makes much difference where the light comes from. People use 1 and pens every day. They have to be very careful about the2of the light because the light shines on their work.
Every 3gets its light either from daylight 4 the window or from the lamps, 5 no matter what kind of light it is, the way it shines is very important to the eyes.
Take a book and sit with your 6 towards the window, and try to read. Your shadow(影子) falls all over the page and makes it 7 for your eyes as if you were in a 8 room. Now turn around and face the window. The page is in shadow again, 9 the bright light is in your eyes.Try sitting with your 10 side towards the window. This is very good for reading, but if you were writing, the shadow 11 fall across the page and trouble you 12.There is another way – sit 13 your left side to the window. Now everything is perfect for 14 as well.
Whatever kind of light is in the room, the rule about the right way to sit is 15 the same. So you see different ways to sit make much difference in writing or studying.
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为电场中的两点．一个离子从
运动到
（不计重力），轨迹如图所示．则下列判断正确的是