根据材料内容，选择正确答案。

    Tom went to see his grandparent with his father. In the train Tom often put his head out of the window. His father said, "Tom, don't put your head out of the window." But Tom went on putting his head out of the window.

    His father took Tom's hat quietly and hid it behind his back and said to Tom, "You see your hat is gone." Tom was afraid. His father said, "Well, whistle(吹口哨) once. Maybe your hat will come back." Tom whistled. His lather put his hat on Tom's head quickly. "Oh! It's wonderful!" Tom laughed and said. He quickly took his father's hat and threw it out of the window. "Now it's your turn to whistle, Dad!" he said happily.

如图（a）所示，用铁架台、弹簧和多个已知质量且质量相等的钩码，探究在弹性限度内弹簧弹力与弹簧伸长量的关系实验．

①为完成实验，还需要的实验器材有：____（游标卡尺或刻度尺）；

②图（b）是弹簧所受弹力F与伸长量x的关系图线，由此可求弹簧的劲度系数k=____N/m．图线不过原点的原因是由于____．

1929-1933年资本主义经济危机爆发的根本原因是（   ）