题目

阅读理解Helen got up at seven thirty. Her parents and her little twin brothers went travelling in Australia and she had to take care of herself at home. She opened the fridge, but she found only three pieces of bread in it. She decided to have breakfast in the restaurant at the other side of the street. She sat at a table and the waitress came up to her and said, “Can I take your order, Helen?”“But you didn't show your menu to me, Miss White,” said the girl.“Oh, sorry! I forget it. Here it is.”Helen looked at the menu carefully. It was:bread $1.5 egg $ 0.4pizza $2.5 cola $ 0.3s如图所示，半径为r的硬橡胶圆环，其上带有均匀分布的正电荷，单位长度的电量为q，其圆心O处的合场强为零，现截去圆环顶部的一小段AB，AB=l（l＜r），则关于剩余部分在圆心O处产生的场强，下列说法中正确的是（　　）A．O处场强方向竖直向上B．O处场强方向竖直向下C．O处场强的大小为klqr2（k为静电力恒量，下同）D．O处场强的大小为kq（2πr-l）r2