根据短文理解，选择正确答案。

    Tea to the Chinese is like coffee to the Americans. It's something that they can't live without.

    Chinese tea has a history of more than 5,000 years. It was most popular during the Tang Dynasty. China has many kinds of tea. White, yellow, green, oolong (乌龙茶), black and dark tea are the six main kinds.

    People in different places drink tea in their own way. People in Guangdong and Fujian love to drink “Kung Fu Tea” in small tea cups. They clean the teapot (茶壶) first with boiling water to warm it up. Then they fill the teapot with tea leaves before putting in the boiling water.

    In Beijing, people love jasmine flower（茉莉花）green tea. In Tibet, butter tea (酥油茶) is a popular drink to build up energy.

    There are rules to follow when we serve tea to others. Most importantly, remember to make the cup only seven-tenths full. If the cup is too full, it makes the cup too hot for them to hold.

如图，一关于y轴对称的导体轨道位于水平面内，磁感应强度为B的匀强磁场与平面垂直。一足够长，质量为m的直导体棒沿x轴方向置于轨道上，在外力F作用下从原点由静止开始沿y轴正方向做加速度为a的匀速加速直线运动，运动时棒与x轴始终平行。棒单位长度的电阻ρ，与电阻不计的轨道接触良好，运动中产生的热功率随棒位置的变化规律为P=ky ${}^{3/2}$ （SI）。求：

①导体轨道的轨道方程y=f（x）；

②棒在运动过程中受到的安培力F_m随y的变化关系；

③棒从y=0运动到y=L过程中外力F的功。

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    ①读书以过目成诵为能，最是不济事。

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