阅读下列材料,从材料后各题所给的A、B、C、D四个选项中,选出最佳选项。
A
Students in Denmark take national tests on a computer at different times. About 4 years' test results, researchers found something interesting. Students who took the test in the morning scored higher than those who took it in the afternoon.
So what happens? We might have noticed that in the afternoon we become less productive(有效益的) and the work we do is of lower quality. A lot of people around the world experience the same thing every day. That's called the afternoon slump(低迷).
Why does this happen? Our bodies are controlled by our circadian rhythm(昼夜规律) which is like a clock. It tells our bodies when to go to sleep and wake up. Our energy levels change with the circadian rhythm.
We are smarter and more energetic during some parts of the day, while during others, we are less energetic. For example, during the night we experience a large drop in energy, especially between 2 a.m. and 4 a.m. However, our energy slowly rises after waking up until it reaches a peak(峰值) in the late morning. But that peak is then followed by an afternoon low, which we call the afternoon slump. The low appears about seven hours after waking up, which is usually between 1 p.m. and 4 p.m. for most people. This is the time when people make more mistakes during the daytime. When the afternoon slump is over, it rises again. But as our body gets ready for sleep, it slowly drops.
The afternoon slump is natural, but there are some ways to regain(恢复) energy. Stretching(拉伸) for even 20 seconds, taking a break for about 15 minutes, eating a bar of black chocolate and so on can have a huge influence on your energy levels. Hopefully, you have a better idea about the afternoon slump and what you can do about it.
() 1. How does the writer support his idea in the third paragraph?
A. By comparing the fact.
B. By explaining the reason.
C. By giving an example.
D. By listing some numbers.
() 2. What can we know about the "afternoon slump"?
A. It only happens to students in Denmark.
B. It makes students score higher in tests.
C. It is caused by natural body rhythms.
D. It's a sign of serious health problems.
() 3. Which picture shows the change of the energy level with the time?

() 4. What can we infer according to the passage?
A. The afternoon slump can be completely avoided by getting enough sleep.
B. Our energy levels are the highest in the late morning.
C. The afternoon slump is a problem that only students face.
D. Simple actions like eating snacks may help get energy back.