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(66)between sending the notification and notification arriving at the source, thousands of additional(67)may arrive. In ATM network, a major tool for preventing(68)is(69)control. When a host wants a new virtual(70), it must describe the traffic to be offered and the service expected.

A．interval
B．time
C．slot
D．delay