题目内容
听力原文： Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the speed of about one-fifth of a mile per second. If the hill is eleven hundred feet away, it takes two seconds for the sound to travel to the hill and back. Thus by measuring the time between a sound and its reflection-the echo, you can calculate the distance to an obstruction.
During World War II the British made a practical use of this rule to discover planes on their way to bomb London long before the enemy was near the object. They used radio waves instead of sound waves, since radio waves can pass through fog and clouds. The outnumbered Royal Air Force (RAF) always seemed to puzzle Germans that they were lying in wait at the right time and were never surprised. It was radio echoes more than anything else that helped them win the war.
Since the radio waves were used to tell the direction in which to send the RAF planes and the distance to send them, the device was called radio directing and ranging, and from the first letters the word radar was formed.
Sound waves reflected from a hill can be used to calculate the______.
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听力原文：M: Hey Lisa, how's it going？
W: Uh, I'm OK, but my schoolwork is really stressing me out.
M: What's wrong？
W: Well, I've got a paper to write, and two exams to study for. And a bunch of math problems to finish. It's just so much that I can't concentrate on any of it. I start concentrating on studying for one of my exams, and then think, how long's it gonna take to finish that problem set？
M: Sounds like you've got a lot more work than you can handle. Have you talked to some of your professors... I mean, try to explain the problem. You could probably get an extension on your paper, or.
W: Then it would give me a little more time to prepare for my exams.
M: Well, have you tried making yourself a schedule？ That's what I do when I'm feeling overwhelmed.
W: What does that do for you？
M: Well, it helps you to focus your energies. You know, you make yourself a chart that shows the next few days and the time till your stuff is due and.
W: Uh-huh.
M: Think about what you need to do, and when you have to do it by, and then start filling in your schedule. But don't make the time periods too long, you'll get tired, or start worrying about your other work again. If you keep to your schedule, you'll just have to worry about one thing at a time.
W: Yeah, that might work.
What are the two speakers mainly talking about？
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