题目

In 1883，an engineer named John Roebling intended to build a bridge connecting New York with the Long Island.  36 ，experts throughout the world thought it impossible and not  37  . Roebling couldn’t  38  the vision in his mind of this bridge. He knew deep in his heart it could be done. He just had to  39  the dream with someone else. After much persuasion he managed to  40  his son Washington，a young engineer，that the bridge  41  could be built. Working together，the father and son developed concepts of  42  it could be accomplished and how the difficulties could be  43运动物体的位移s=gt2（g=9.8 m/s2），则此物体在t=2 s时的瞬时速度为（　　）A.19.6 m/sB.9.8 m/sC.4.9 m/sD.39.2 m/s