There is a new park near Andy's home. It's fine today. Andy and his family are in the park now. On their left, there is a café. On the right, there is a big lake. There are many flowers and trees around the lake. There's a small hill behind the lake. Near the lake, there are two signs（标记）. One says, "Don't swim in the lake."But you can go boating on the lake. There is a beautiful garden in the middle of the park. There are green grass and beautiful flowers in it. There are some small shops between the lake and garden. The park is so nice. Andy and his family like it very much.

两个等量点电荷P、Q在真空中产生电场的电场线（方向未标出）如图所示．下列说法中正确的是 (    )