My name is Tommy Wang. T-O-M-M-Y, Tommy, W-A-N-G, Wang. I am a Chinese boy. My telephone number is 85523963. My telephone is white. Look! This is my pencil box. ① It's big and beautiful. It's white. What's in it? This is ② ________ eraser. It's white. This is a pencil. It's white. ③ This is a ruler. It's white, too. I like white. I like green, too. Look! ④ My schoolbag is green. My notebook is green, too. But my favorite color is white.

    This is my good friend. His name is John. ⑤ He is English. Linda is John's sister. I like my friend. Like English.

如图所示，两竖直放置的平行光滑导轨处于垂直于导轨平面的匀强磁场中，金属杆ab可沿导轨滑动，原先S断开，让ab杆由静止下滑，一段时间后闭合S，则从S闭合开始计时，ab杆的运动速度v随时间t的图象不可能是下图中的（   ）