5+7+9+11+13+15+17的结果是( )
A:81
B:72
C:77
Dear Alan,
I'm looking forward to staying in the countryside with you for one week this summer vacation.
Yours truly
Li Hua
如图所示,物体A在力F的作用下,以0.5m/s的速度在水平桌面上做匀速直线运动,弹簧测力计示数为2N,不计滑轮和绳重以及它们之间的摩擦,则物体A受到的摩擦力是____ N,5s内拉力做的功是____ J,拉力的功率是____ W.