A、Because the speed of light changes easily. B、Because light slows down when it enters other medium. C、Because light moves very

If you’re driving on a highway, the only thing keeping you from going faster than the speed limit is your sense of civic duty, or perhaps that police car up ahead. There’s one speed limit, however, that most scientists agree nothing will ever break. That’s the speed of light: 186, 280 miles per second. As it trav-els through deep space, light moves at exactly this speed, no more and no less. The speed of light is so reliable, we even use it to measure distances between the stars. If it takes starlight fifty years to reach us, we say that this star is fifty light years away. You might be surprised to learn, however, that light itself does not always travel at the speed of light. Our highways generally have a higher speed limit, but once you enter a city or residential street, the speed limit lowers. Likewise, light travels at its maximum speed through a vacuum but it slows down when it enters air, water, or some other medium. Through air, light moves a little slower: forty miles per second less than its maximum speed. Through glass, light moves almost eighty thousand miles per second slower.Light moves more than a hundred thousand miles per second slower through a diamond less than half its speed in a vacuum. Light moves so quickly anyway, do we ever notice this change in speed? Actually, we do. When light slows down it also bends slightly. If light always moved at its maximum speed, none of our telescopes, eyeglasses or even our eyes themselves would ever work.
1.What does the passage mainly talk about?
2.Why do we use light speed to measure distances between the stars?
3.What will happen if light moves through a diamond?
4.What would happen if light always moved at its maximum speed?