In low-speed network，it is usually adequate to wait for congestion to occur and then react to it bytelling 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 offeredand the service expected．

（73）处填（）。

A．collision

B．congestion

C．drop

D．delay

椐实验研究，视网膜完全缺血多长时间后出现不可逆损害（）

A.60分钟

B.90分钟

C.30分钟

D.150分钟

E.120分钟