质谱仪是一种测定带电粒子质量或分析同位素的重要设备，它的构造原理如图示．离子源S产生的各种不同正离子束（速度可视为零），经MN间的加速电压U加速后从小孔S₁垂直于磁感线进入匀强磁场，运转半周后到达照相底片上的P点．设P到S₁的距离为x，则（   ）

    Early rising is helpful in more than one way. First, it ____ us to have good health. We all need fresh air. It makes our health better to take morning exercise. But air is never so fresh as early in the morning. Early rising helps us in our studies. In the morning we learn ____ and find it easy to learn something ____ heart. Early rising makes people able to plan the work for the day. We can not work well without a good plan. Just as, "The plan for a year should be made in spring, the plan for a day should be made in the morning." So the popular ____ says, "Early to bed and early to rise makes man ____, wealthy and wise."