阅读短文，从方框中选出适当的单词填空

Dear Ricky,
    How are you ____ on? Thanks very much for sending me the pictures and the English reading materials. From the pictures I can ____ that your school looks very beautiful. The reading materials are very useful and ____. I can learn a lot from them.  Some stories are so interesting that I like them very much.
    Now I don't have ____  time for my hobbies. I am busy getting ready for the coming exams. I’m working hard at all my lessons because I want ____ a better result.
    I'm looking forward to hearing from you soon.
Yours,
Wu Ming

下列有关科学家的经典研究中，采取了同位素标记法的是（    ）
① 恩格尔曼发现光合作用的部位           
②梅耶指出植物通过光合作用把光能转换为化学能
③鲁宾和卡门证明光合作用释放的 O2 来自水
④卡尔文探明了 CO2 中的碳在光合作用中转化成有机物中碳的途径

材料与用具：细胞培养瓶、小鼠肝癌细胞悬液、肝癌细胞培养液、不同浓度的药物X溶液（用生理盐水配制）、生理盐水、活细胞检测仪等．

如图所示，在xoy坐标系中，y＞0的范围内存在着沿y轴正方向的匀强电场，在y＜0的范围内存在着垂直纸面的匀强磁场（方向未画出）．已知oa=oc=cd=L，ob= $\frac{L}{4}$ ．现有一个带电粒子，质量为m，电荷量大小为q（重力不计）．t=0时刻，这个带电粒子以初速度v₀从a点出发，沿x轴正方向开始运动．观察到带电粒子恰好从d点第一次进入磁场，然后从O点第﹣次离开磁场．试回答：