写出含有字母x、y的四次单项式______（只要写出一个）．

阅读理解。

     Do you know why you sleep? You may think it's because you're tired. Or perhaps you've been told sleep

is good for you. Yet no one knows exactly why or how people sleep.

      Until the early 1950s, most scientists thought the body and brain rested during sleep. Then a scientist

watched people sleep. He noticed that during periods of between 5 and 30 minutes, sleepers' eyes made

rapid movements, as if they were watching something. He called these periods REM (Rapid Eye Movement)

sleep. He called periods when sleeper's eyes did not move non -REM sleep.

     Whenever the scientist woke people during REM sleep, they were having vivid dreams. Measurements

showed that sleepers' pulses (脉搏) went up quickly during REM sleep. Their breathing, heartbeat, and the

blood going to the brain also increased. Far from being a time of rest, sleep is a time when the body is hard

at work. Scientists still don't know the reason for this. Some believe REM sleep makes learning easier.

Babies spend a lot of time on REM sleep. Because of this, some scientists believe that REM sleep helps

the brain grow. Others think REM sleep is a time for the brain to empty the day's garbage.

      One thing all the scientists agree on is that sleep is necessary. Without enough sleep, people make

mistakes. Most car accidents happen during the early morning hours, when people are sleepiest.

1. You may read the passage in _____. [ ]

A. an advertisement

B. a history book

C. a science book

D. a telephone book

2. During REM sleep, a person's breathing and heartbeat _____.　 [ ]

A. stop

B. increase

C. sleep

D. stay the same.

3. The underlined word "vivid" in the passage means _____. [ ]

A. 简单的

B. 空洞的

C. 生动的

D. 鲜艳的

4. What is the main idea of the article? [ ]

A. REM sleep has been observed by all scientists.

B. REM sleep helps the brain grow.

C. REM sleep is a time to empty the day's garbage.

D. REM sleep is an active time when the body is hard at work. 

5. We can know from the article that REM sleep ._____ [ ]

A. has an effect on people.

B. can be stopped with lots of sleep.

C. is something that all people experience.

D. is something that all scientists agree on.

某研究者为了验证氯霉素对伤寒的疗效，在408例伤寒患者中进行对照实验，其中251例用氯霉素治疗，其余157例不用。结果使用组251人中死亡20人，病死率7．07％；未用组157人中死亡36人，病死率22．8％。

在人体实验中使用对照组、安慰剂和双盲法（）

A.是对病人的一种欺骗

B.是违背人道主义原则的

C.是违背知情同意原则的

D.以征服疾病为目的，为发展医学，在严格控制并不损害患者利益的范围之内使用这一方法，是合乎医学道德的

E.是医学科研中经过考验的正确方法，可以在人体实验中广泛应用

-How was your school trip?

-It was great,         the weather was much too hot.

A．and

B．but

C．so

D．because

边检人员在与不熟悉的服务对象接触时，眼睛应看向服务对象面部的（）。

A、大三角

B、小三角

C、倒三角

D、正三角

仓储信息管理系统登录新建用户时，下面哪些不能做登录名（）

A.数字

B.字母

C.文字

D.以上均可