In the following essay, each blank has fou
题型：单项选择题
问题：
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.
2（）
A．packets
B．cells
C．message
D．files