某单位通过2Mbps的DDN专线接入广域网，网络拓扑结构如图6-5所示。该单位申请的公网IP地址为61.246.100.96/29。其中，该单位能够使用的有效公网地址有 (70) 个。

单项选择题The spiral model is a software development model combining elements of both (73) and prototyping-in-stages, so it's a healthy mix of top-down and bottom-up concepts。

单项选择题In (71) programming, the user determines the sequence of instructions to be executed, not programmer。