阅读理解。
●What prevents depression; tea, coffee or hot chocolate?
Women who drink for or more cups of coffee a day are a fifth less likely to become depressed.
Drinking two or three cups reduces the risk by 15 percent. This is the result of a study of 51,000 women
over ten years, published in the Journal of the American Medicine Association.
One theory is that caffeine reduces the effect of the chemical adenosine (腺苷) in the brain, which
can make us feel sleepy by slowing down nerve cell activity. Some anti-depressants also work by
blocking the adenosine receptors. Coffee was found to be far more influential on depression levels than
hot chocolate or tea due to the high caffeine content.
●How many eggs can we healthily eat: one a day or two a day?
Neither-eat them freely. We've long been warned to stick to two or three eggs a week because of
their high cholesterol (胆固醇) content. But according to the Food Standards Agency, there is no
recommended limit to how many eggs we can healthily eat, unless you have been told to cut down by
your doctor.
There is no evidence that eating eggs raises cholesterol levels, and researchers found that eggs contain
less cholesterol than in the past because hens are no longer given bone meal, which was banned in the
1990s after the BSE (疯牛病) crisis.
Research by the British Egg Council found that a medium egg gives you around 100 mg of cholesterol, a third of the 300mg recommended daily limit.
1. What is considered to fight depression best?
A. Tea.
B. Chocolate.
C. Adenosine.
D. Coffee.
2. We can infer from the text that __ .
A. too much caffeine will probably make people sleepy
B. coffee contains less caffeine than tea or hot chocolate
C. one is depressed if his brain chemical adenosine is active
D. tea and hot chocolate have no effect on men's depression
3. What can we learn about eggs from the text?
A. We should stick to two or three eggs a week in general.
B. Eat as many eggs as you like unless your doctor advises you not to.
C. Eggs contain more cholesterol at present than ever before.
D. One egg daily and you'll get more cholesterol than recommended.
乙车速
则t2后乙车在前面，设再经过Δt甲追上乙，有
解得：Δt＝4s，此后，乙不能再追上甲，故甲乙相遇三次。
解得t5=4s。设第三次等速经过的时间为t6，则
解得：t6=8s