（14分）如图甲所示，空间存在B=0.5T、方向竖直向下的匀强磁场，MN、PQ是相互平行的粗糙的长直导轨，处于同一水平面内，其间距L=0.2m，R是连在导轨一端的电阻，ab是跨接在导轨上质量m=0.1kg的导体棒，从零时刻开始，通过一小型电动机对ab棒施加一个牵引力F，方向水平向左，使其从静止开始沿导轨做加速运动，此过程中棒始终保持与导轨垂直且接触良好，图乙是棒的速度—时间图像，其中OA段是直线，AC是曲线，DE是曲线图像的渐近线，小型电动机功率在12s末达到额定功率P_m=4.5W，此后功率保持不变，除R以外，其余部分的电阻均不计，取g=10m/s²。求：

（1）导体棒在0～12s内的加速度大小；

（2）导体棒与导轨间的动摩擦因数和电阻R的阻值；

（3）若已知0～12s内R上产生的热量为12.5J，则此过程中牵引力F做的功。

One of the most widely used routing protocols in IP networks is the Routing Information Protocol (RIP)．RIP is the canonical example of a routing protocol built on the (71) algorithm．Routing protocols in internet works differ slightly from the idealized graph model．In an internet work，the goal of the routers to forward (72) to various networks．
Routers running RIP send their advertisement about cost every (73) seconds．A router also sends an update message whenever an update from another router causes it to change its routing table．
Routing protocols use different techniques for assigning (74) to individual networks．Further, each routing protocol forms a metric aggregation in a different way．RIP takes the simplest approach，with all link costs being equal (75) ．Thus it always tries to find the minimum hop route．Valid distances are 1 through 15．This also limits RIP to runing on fairly small networks．

A．data frames
B．segments
C．packets
D．messages