完形填空。
阅读下面短文，从短文后各题所给的四个选项（A、B、C和D）中，选出可填入空白处的最佳选项。
As I walked down the hall on the last day of the school year, I watched wide-eyed as my fellow students 1their drawers into the dustbins in front of the hall. Paper flew in all directions; broken shelves were 2 everywhere.
Suddenly, I came up with an idea: my school really needed a 3 programme. I believed that one person could make a 4so I wrote out my recycling5 As I told the schoolmaster about my 6 my heart was beating wildly.
With her agreement and encouragement, I would 7 cardboard boxes in the library, offices and classrooms to 8 papers, magazines and newspapers.
That night, I designed a dozen 9and planned to arrive at school early to stick them up and hand out 10to classrooms on Monday. But when I arrived, I was very disappointed－the night-watchman had 11down all of the cardboard boxes I had saved. I was so upset. I began searching for other boxes, but I met with 12 success.
A few days later,13 my English teacher took me into the new computer lab, and before my eyes sat a 14of cardboard boxes. I am sure I was the 15 person in the world when I saw these boxes. These were the boxes that the new computers had arrived in, and they were the16size.
I rolled my recycling rubbish bins to the dining room, stuck up my posters, and17made an announcement18 the loudspeaker, letting the whole school know they now had a(n)19 about what would happen to their unwanted papers, cans and bottles.
Since that day, recycling has become part of everyday life at my school. Students and teachers now take active 20 to save trees and other natural resources.
的图象与x轴交于A、B两点， A点在原点的左侧，B点的坐标为（3，0），与y轴交于C（0，-3）点，点P是直线BC下方的抛物线上一动点.
，那么是否存在点P，使四边形
为菱形？若存在，请求出此时点P的坐标；若不存在，请说明理由．