Communication protocols are (66) connection-oriented or connectionless, (67) whether the sender of a message needs to contact and maintain a dialog with the recipient or (68) send a message without any, prior connect and with the hope that the recipient receives everything. (69) .These methods (70) the two ways that communication is implemented on networks.

A．make known

B．disclose

C．reveal

D．discover

网管及支撑系统维护工作主要包括系统本身及相关设备的日常维护、安全管理、（）、故障处理等。