①相应的骨受到牵引，

②骨绕关节转动，

③骨骼肌接受神经传来的兴奋，

④骨骼肌收缩．这些步骤发生的正确顺序是

   As a child, I was really afraid of the dark and of getting lost. These fears were very real and caused me some uncomfortable moments.
Maybe it was the strange way things looked and sounded in my own room at night that scared me so much. There was never complete darkness, but always a streetlight or passing car lights, which made clothes on the back of a chair take on the shape of a wild animal. Out of the corner of my eye, I saw the curtains(窗帘) seem to move when there was no wind. A very low sound in the floor would seem a hundred times  louder than in the day. My imagination (想象) would run wild, and my heart would beat fast. I would lie very still so that the "enemy(敌人) " would not discover me.
   Another of my childhood fears was that I would get lost, especially on the way home from school. Every morning I got on the school bus right near my home. That was no problem. After school, though, when all the buses were lined up along the street, I was afraid that I would get in the wrong one and be taken to some other strange places. On school or family trips to a park or a museum, I wouldn't let the leaders out of my sight.
   Perhaps one of the worst fears of all I had as a child was that of not being liked or accepted by others. Being popular was so important to me then, and the fear of not being liked was a serious one.
   One of the processes(过程) growing up is being able to realize and overcome our fears. Understanding the things that scared us as children helps us achieve greater success later in life.

某物体做匀速直线运动，它在30s内通过的路程是150m，它的速度是 ____m/s，它15s内通过的路程是 ____m，它在第40s内的速度是 ____m/s。