将多台PC机组成以太局域网时，需要一些连接设备和传输介质。下面哪个设备或介质是不需要

若一棵二叉树中只有叶节点和左、右子树皆非空的节点，设叶节点的个数为k，则左、右子树皆非空的节点个数是 【4】 。

期货经纪公司客户的开户资料应至少保留（）。

A．3年

B．5年

C．10年

D．20年

阅读理解。

     Suppose a key man in your firm had just met with a terrible accident. The doctor tell you that this man

upon whom you depend for directing sales, checking your books or for performing some other important

roles, will be laid up for months. You'd have to replace him, wouldn't you? And probably at a pay about

equal to his?

     Then you'd be faced with double paying for one job because stopping the pay of an injured man would

simply be out of the question.

     You can prevent this kind of unproductive outgo (支出) on your payroll (薪水册) by providing your

key man with the protection of Travelers Business Accident Insurance!

     This insurance provides total medical expense and a weekly income in case a key man is disabled for

some time. Should the disability prove everlasting, an income for life is certainty. Why not make out a list

of your key men now? Then get together with Travelers Insurance Company, and let us tell you how little

it costs to apply this much needed protection to your business.

     Write and we'll serve you!

1. The above is most probably _____.

A. a report

B. a letter

C. an announcement

D. an advertisement

2. What is really true according to the above?

A. Travelers Insurance Company provides services of accident insurance.

B. An important person of the firm has just had an accident.

C. Accident Insurance will protect the important person from accident.

D. Unfortunately the important person has become disabled.

3. Whom might the writing be mainly for?

A. An important person in a firm.

B. An employee of a firm.

C. The boss of a company.

D. Readers of a newspaper.

4. Why would the pay be doubled?

A. Because two persons are doing one job of the same.

B. Because the important job is worth double pay.

C. Because it is necessary that the injured person should be paid.

D. Because the double pay is for two persons.

5. Travelers Insurance Company _____ for a person insured.

A. will pay for all the cost of treatment

B. will pay the weekly income or even that of lifetime

C. won't get much from the firm

D. will provide everlasting pension as well as medical expense

如果脂溶性药物透过膜的速度与跨膜浓度梯度以及油水配系数成正比时，这属于的跨膜转运机制是（）

A.滤过

B.被动扩散

C.易化扩散

D.主动转运

E.吞饮作用

北京民居建筑的代表是（  ）

A．高架屋

B．四合院

C．竹楼

D．蒙古包