（2015•绵阳）关于电磁波，下列说法正确的是（　　）

静电场方向平行于x轴，其电势随x的分布可简化为如图所示的折线，图中φ₀ 和d为已知量．一个带负电的粒子在电场中以x=0为中心、沿x轴方向做周期性运动．已知该粒子质量为m、电量为﹣q，忽略重力．规定x轴正方向为电场强度E、加速度a、速度v的正方向，如图分别表示x轴上各点的电场强度E，小球的加速度a、速度v和动能E_k随x的变化图象，其中正确的是（   ）

    English class is always full of fun. It's a tradition to put on an English play at the end of the year. Yesterday our English teacher gave us copies of the play.

    "Choose a character in the play whom you'd like to be," she told us. "This afternoon you will learn a few lines (台词) of this part for homework. Then tomorrow you can say those lines in front of the class. I'll decide who plays each part."

    I wanted to have one of the biggest parts in the play, so I chose the part of the king. He has a lot of lines to say. I learnt them by heart.

    This morning in our English class we had to say our lines. I acted the part of the king. When l was saying my lines, I put a lot of expressions into my acting. I thought I acted really well.

    Many of the students in my class couldn't act at all. They couldn't even read without making lots of mistakes. I was sure that l would get a big part in the play.

    However, the teacher gave me the part of a soldier. He has only one line. It is, "Yes, sir!"

    I was very disappointed. At the end of the lesson I went up to her and asked her why I had such a small part.

    "You put too many expressions in your acting," she said. "Good acting is not big acting. It is acting the audience can believe in." I think I know what she means.