Today is Tree- Planting Day. I go to the park with my parents. At about ten o'clock we start to plant trees. We needn't to dig holes because the people in the park dug them before. We just put the young trees into the holes. My father fills the holes with me. My mother carries water for the young trees. We are so tired when we finish the work. All of us hope the trees will grow well. I hope I can plant trees next year.

从上表可知，下列说法中正确的有（   ）

① $\frac{c}{a}$ =6；②函数y=ax²+bx+c的最小值为﹣6；③抛物线的对称轴是x= $\frac{7}{2}$ ；④方程ax²+bx+c=0有两个正整数解．

如图所示，边长为L的正方形闭合线框，在磁感应强度为B的匀强磁场中，以一条边为轴，以角速度ω匀速转动，转轴与B垂直，线圈总电阻为R，导线电阻不计．下列说法中正确的是（   ）

牛顿第一定律表明，力是物体____发生变化的原因；该定律引出的一个重要概念是____。