发作性呼吸困难，临床上最常见的原因是（） A.自发性气胸 B.支气管内异物或阻塞 C

搪塑

传统的交换机作为第二层设备，只能识别并转发 (1) 地址，要支持VLAN间的通信只能借助于具有 (2) 功能的网络设备。具有这种功能的设备有路由器和三层交换机。当用路由器解决VLAN通信时，路由器得到一个VLAN包后，根据目的IP地址，获得目的MAC地址及相应的 (3) ，根据 (4) 将帧送往目的交换机端口。当用三层交换机解决VLAN通信时，由于交换机具有 (5) ，交换和路由速度可以达到线速。

4（）

A．MAC

B．IP

C．VLAN ID

D．端口

Personality is to a large extent inherent. A-type parents usually (1) A-type offspring. But the environment must also have a profound effect (2) if competition is important to the parents, it is (3) to become a major factor in the lives of their children.

One place where children (4) A characteristics is school, which is, (5) its very nature, a highly competitive institution. Too many schools (6) the "win at all costs" moral standard and measure their success by sporting achievements. The current (7) for making children compete against their classmates or against the (8) produces a two-layer system, in which competitive A types seem in some way better than their B type fellows. Being too (9) to win can have dangerous consequences: remember that Pheidippides, the first marathon runner, (10) dead seconds after saying. "Rejoice, we conquer!"

(11) the worst form of competition in schools is the disproportionate emphasis on examinations. There is, for example, a (12) school that allows pupils to concentrate on those things they do well. The (13) of competition by examination are somewhat questionable, but competition in the certain (14) of failure is positively harmful.

(15) , it is neither practical nor desirable that all A youngsters change into B’s. The world needs types, and schools have an important duty to try to (16) a child’s personality to his possible future employment. It is top management.

If the preoccupation of schools with academic work was (17) , more time might be spent teaching children surer values. Perhaps selection (18) the caring professions, especially medicine, could be made less by good grades in chemistry and more by such (19) as sensitivity and sympathy. It is surely a mistake to choose our doctors exclusively from A type stock. B’s are important and should be (20) .

6（）

A.adapt

B.abide

C.adopt

D.absorb

OSI参考模型共分为7层期中数据链路层是第（）层。

A、2

B、3

C、4

D、5

目前梨果皮颜色的理想育种方向是（）。

A.红色

B.褐色

C.黄色

D.棕色