In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

In low - speed network, it is usually adequate to wait for congestion to occur and then react to it by telling the source of packets to slow down. In high - speed networks, this approach often works poorly, because in the (1) between sending the notification and notification arriving at the source, thousands of additional (2) may arrive. In ATM network, a major tool for preventing (3) is (4) control. When a host wants a new virtual (5) , it must describe the traffic to be offered and the service expected.

4（）

A．flow

B．admission

C．traffic

D．time

小肠绒毛的壁很薄，只由一层什么结构组成

A．肌肉组织

B．上皮组织

C．肌肉细胞

D．上皮细胞