用毡席材料作施工绝热层时，捆扎时环向间隔（）捆扎铁丝或箍带一道。
A．≤150mm
B．＞300mm
C．150～300mm
D．不一定
参考答案：D
. Suddenly everyone stopped talking. It became clear that something unexpected _____. But, for a while, I didn’t know what it was.
A．has happened
B．is happening
C．happened
D．had happened
(10分)在水平地面上有一质量为2kg的物体，物体在水平拉力F的作用下由静止开始运动，10s后拉力大小减为F／3，该物体的运动速度随时间t的变化规律如下图所示．
求：（1）物体受到的拉力F的大小．（2）物体与地面之间的动摩擦因数．(g取10m／s2)
