切削速度大于35m/min时，（）。 A.切削力增大 B.切削力减小 C.切削力不变

一列简谐横波在x轴上传播，在t₁=0和t₂=0.05s时刻，其波形图分别如图中的实线和虚线所示，求：

（1）该波的振幅和波长？

（2）若这列波向右传播，波速是多少？若这列波向左传播，波速是多少？

（3）当波速为280m/s时，波的传播方向？

Thanks for giving me ______ advice, but it’s difficult ______ me to carry it out.

A．such a good; for

B．so good; for

C．such good; for

D．so good a; for

北京市所处地形区主要是（　　）

A．东北平原

B．华北平原

C．长江中下游平原

D．黄土高原

阅读理解。

     Many researchers used to think yawning (打呵欠) was necessary for increasing oxygen intake to the

lungs (肺), while others insisted it was to increase blood pressure and the heart rate.

     While both findings are correct, new studies show that the main point of yawning is to relax.

Researchers point out that we have a tendency to yawn before big events-athletes yawn before races, pilots

yawn before take-offs, and students yawn before exams. Humans are not the only creatures who yawn.

Animals have been observed to yawn before feeding time. while wild animals have a tendency to yawn

before fights.

     New findings also show that yawing might be a handover from our ancestors, and represents a need

for change. Our ancestors passed it to us with other life skills. Even babies yawn as they change positions,

but children do not begin to copy other people's yawns until the age of four or five.

     Strangely enough yawning is also related to respect. Hundreds and thousands of years ago,the campfire

(篝火) served as the main gathering point for members of groups. Researchers claim that as people sat

around the fire, warming themselves, they would yawn if the leader yawned, as a mark of respect. Those

who were more supportive of the leader tended to yawn more easily.

     Despite all these findings, scientists still haven't figured out why we yawn before bedtime. Then again,

it may be another way of demanding a change-enough of TV, it's time to go to bed!

1. What is the text mainly about?

A. People who tend to yawn.

B. Why we yawn.

C. The history of yawning

D. The disadvantages of yawning.

2. According to researchers, yawning before an exam suggests that _____.

A. you need to relax.

B. you don't like taking exams

C. you want to control your heart rate

D. you're sleepy

3. The underlined word "handover" in the third paragraph probably refers to _____.

A. a need for change

B. something our ancestors have passed on to us

C. the way in which people copy others'yawns

D. a lost life skill

4. From the fourth paragraph we can leam that _____.

A. people used to yaxvn to draw attention

B. people yawn when they feel warm

C. yawning happened at night in ancient times

D. yawning was a social skill in ancient times