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．

（72）处填（）。

A．packets

B．cells

C．message

D．files

上市开放式基金份额的转托管业务包含两种类型：系统内转托管和跨系统转托管。（　　）