题目内容：
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（71）between sending the notification and notification arriving at the source,thousands of additional（72）may arrive.In ATM network,a major tool for preventing（73）is（74）control.When a host wants a new virtual（75）,it must describe the traffic to be offered and the service expected.
A.packets
B.cells
C.message
D.files
参考答案：
答案解析：