阅读下列短文，从下面每小题的A、B、C、D四个选项中选出最佳选项。
My experience as a volunteer was really exciting and useful. That afternoon we arrived at La Gran Vista Farm. Donald, the man who was in charge of the farm, Xenia, his wife, Terry, Max, and Larry, their 3 sons, were very friendly to us as soon as we got there. They told us what we were going to do, and we had a delicious meal with them. Within minutes I found that La Gran Vista Farm was a special place. The farmhouse was a beautiful hand-made building. It could hold about 20 volunteers.
The land was very beautiful. For the first week we worked on the farm. In the morning, Donald would divide us into 7 groups. We did the things from planting trees, cleaning the road, to taking out the rubbish. A workday was as follows: 6:30 get up, 7:00 breakfast, 8:00-11:30 work, 12:00 lunch, 13:00-15:00 work, 17:00 dinner.
Donald would come around from group to group and explain what we were doing and why. He wanted to set up a farm that was not bad for the environment. The program made me understand the importance of protecting the environment. And it took me to a new culture. I will never forget my volunteering experience on the La Gran Vista Farm. It taught me a lot.
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，当分子力表现为斥力时，r必须满足（ ）
