题目内容：

    We take showers and use the toilet every day to get rid of badly waste. Our brains also need to take a bath every day. How can they do it?

A 2013 study found the answer. Researchers at the LS University of Rochester studied the brains of mice and discovered that they cleaned themselves while the mice slept. A kind of fluid in the brain, called cerebral spinal fluid (CSF, 脑脊液), was found to increase dramatically (显著地) during sleep, washing away waste proteins that had been building up between brain cells in waking hours.

"This study shows that the brain has different functional stales when asleep and when awake," Maiken Nedergaard, the lead researcher, told NBC News. It also explains why we can't seem to think clearly after a sleepless night while a good night's sleep leaves us feeling sharp and refreshed.

Now a new study, published on Oct 31 in Science, digs a little deeper into our brains' self-cleaning procedure. Instead of mice, this time humans were the test subjects.

Researchers at Boston University, US, monitored (监测) the brain waves of 13 healthy adults who were sleeping, using accelerated MRI, which is capable of recording faster changes inside the brain than a regular MRI machine. They found that every 20 seconds, blood flowed out of the brain, making room for a large amount of CSF to come in and "clean". This cycle coincided with(与……一致) the rhythm of the brain's slow waves-an electrical activity that happens when we're in deep sleep.

It's still unknown how these brain activities are connected. But the mere fact that they are connected is exciting enough, since it allows researchers to piece together (拼凑出) possible new explanations for misunderstood diseases.

For example, slow-wave sleep has been proven to play a role in strengthening our memories. This may explain why people with Alzheimer's often have fewer and weaker slow brainwaves. Based on this new study, there could be one more explanation for diseased brains: They are not clean.

Nedergaard, leader of the 2013 study, is also excited about the new findings. "Maybe the most important take-home message is that sleep is a serious thing," she told Scientific American. "You really need to sleep to keep a healthy brain because it links electrical activity to a practical 'housekeeping' function."

1.When the researchers studied the brains of mice, they found __________.

A.CSF in their brains went down when the mice slept

B.CSF carried away waste proteins in their brain during sleep

C.more waste proteins were produced in their brains when asleep

D.the more CSF was in their brains, the fewer waste proteins were

2.What did the researchers discover when they monitored human brain waves?

A.The rhythm of the brain was slower than that of CSF.

B.The brains worked more slowly when asleep.

C.Brains were cleaned when blood flowed out.

D.It took 20 seconds for CSF to clean brains.

3.Which of the following would be the best title for the passage?

A.Connecting Brain Activities B.Cleaning in Your Dreams

C.Having a Good Sleep D.Explaining Diseases