如图，两光滑平行金属导轨置于水平面（纸面）内，轨间距为l，左端连有阻值为R的电阻．一金属杆置于导轨上，金属杆右侧存在一磁感应强度大小为B、方向竖直向下的匀强磁场区域．已知金属杆以速度v₀向右进入磁场区域，做匀变速直线运动，到达磁场区域右边界（图中虚线位置）时速度恰好为零．金属杆与导轨始终保持垂直且接触良好．除左端所连电阻外，其他电阻忽略不计．求金属杆运动到磁场区域正中间时所受安培力的大小及此时电流的功率．

下列各项中，没有语病的一项是(　　)

阅读下列短文,从每小题所给的A、B、C、D四个选项中选出一个最佳选项。

Dear Bill

    Thank you for your letter（信件). You tell me something about your family. Now I'll talk about my family.

    I have a happy family. We love each other. My grandparents are doctors. They work in Canada now. My parents are both teachers. They teach English at Luoyang International School. My sister and I are in Luoyang with them.

    I'm 13 years old. I'm a student in Grade Seven. I like my teacher and my friends in China . They often help me. I want to be a Chinese doctor when I grow up . Now I can speak Chinese. My sister, Jane, is only three years old. She is very cute . She has brown hair and blue eyes. We all love her.China is very beautiful. Welcome to China.

Yours

Frank

我国境内已知的最早人类是（   ）