阅读下列短文，从每小题的四个选项中选出最佳选项

A

      A month ago, the Greens went along the coast and finally got to Daytona beach (代托纳海滩) to spend their holiday. It was 200 miles away from their home in Florida. They took their pet cat with them. The cat got frightened because of some noise during the holiday. Then it ran away. The Greens looked for their pet for a long time,but they didn't find it. They had to leave for home. In fact, the cat traveled for a long time and got back to Florida after it was lost.

     Luckily, a woman called Sarah in Florida found the cat in her garden.The cat was too weak to make a sound, lying there only. When the pet doctor examined the cat and found a chip (芯片) inside the cat's body. And there was all kinds of information about the cat in the chip. Sarah knew that the Greens were the owner of the cat. Sarah took it home and looked after it quite well. The cat became better after a few days.

     Sarah wanted to sent the cat back to the Greens. However, how did the cat get home far away from 200 miles? Maybe only the cat itself knew it.

在正电荷Q的电场中的P点放一点电荷，其电荷量为+q，P点距离Q为r，+q所受的电场力为F．则P点的电场强度为（   ）