游客属于旅行者，但旅行者不都是游客。(　　)P4

待测电阻R_x的阻值约为20Ω，现要测量其阻值，实验室提供器材如下：

A．电流表A₁（量程150mA，内阻约为10Ω）

B．电流表A₂（量程20mA，内阻r₂＝30Ω）

C．电压表V（量程15V，内阻约为3000Ω）

D．定值电阻R₀＝100ΩE．滑动变阻器R₁，最大阻值为5Ω，额定电流为1.0A

F．滑动变阻器R₂，最大阻值为5Ω，额定电流为0.5A

G．电源E，电动势E＝4V（内阻不计）

H．电键S及导线若干

（1）为了使电表调节范围较大，测量准确，测量时电表读数不得小于其量程的，请从所给的器材中选择合适的实验器材     _______________________（均用器材前对应的序号字母填写）；

（2）根据你选择的实验器材，请你在虚线框内画出测量R_x的最佳实验电路图并标明元件符号；

（3）待测电阻的表达式为R_x＝________________，式中各符号的物理意义为________________

______________________________________________________________________________________。

阅读理解。

     Compassion is a desire within us to help others. With effort, we can translate compassion into action. An

experience last weekend showed me this is true. I work part-time in a supermarket across from a building for

the elderly. These old people are our main cus- tomers,and it's not hard to lose patience over their slowness.

But last Sunday, one aged gentleman appeared to teach me a valuable les- son. This untidy man walked up to

my register (收款机 ) with a box of biscuits. He said he was out of cash (现金), had just moved into his

room, and had nothing in his cupboards. He asked if we could let him have the food on trust. He promised to

repay

me the next day.

     I couldn't help staring at him. I wondered what kind of person he had been ten or twenty years before,

and what he would be like if luck had gone his way. I had a hurt in my heart for this kind of hu- man soul, all

alone in the world. I told him that I was sorry, but store rules didn't allow me to do so. I felt stupid and unkind

saying this, but I valued my job.

     Just then, another man, standing behind the first, spoke up. If anything, he looked more pitiable."Charge it

to me," was all he said.

     What I had been feeling was pity. Pity is soft and safe and easy. Compassion, on the other hand, is caring

in action. I thanked the second man but told him that was not allowed either. Then I reached into my pocket

and paid for the biscuits myself. I reached into my pocket because these two men had reached into my heart

and taught me compassion.

1. The aged gentleman who wanted to buy the biscuits___.

A. promised to obey the store rules

B. forgot to take any money with him

C. hoped to have the food first and pay later

D. could not afford anything more expensive

2. Which of the following best describes the old gentleman?

A. Kind and lucky.

B. Poor and lonely.

C. Friendly and helpful.

D. Hurt and disappointed.

3. The writer acted upon the store rules because____.

A. he wanted to keep his present job

B. he felt no pity for the old gentleman

C. he considered the old man dishonest

D. he expected someone else to pay for the old man

4. What does the writer learn from his experience?

A. Wealth is more important than anything else.

B. Helping others is easier said than done.

C. Experience is better gained through practice.

D. Obeying the rules means more than compassion.

大多数OSAS患者睡眠中猝死的主要原因是（）。

A.脑缺氧

B.心律失常

C.呼吸性酸中毒

D.低氧血症

E.高血压

肾小管和集合管上皮细胞分泌NH₄⁺和NH₃的主要生理意义是

A．完成细胞内代谢

B．排泄体内毒素

C．维持机体酸碱平衡

D．维持机体电解质平衡

天然水中离子都是无机盐类溶于水后电离形成的。（）