A、They want to get clean and healthy fruits and vegetables. B、They don’t want to depend on farmers in rural areas anymore. C、The

　　For many years, people in American cities have depended on farmers in rural areas to grow fruits and vegetables. But now a new generation of farmers is planting crops in urban areas. Sean Conroe is a college student. Amber Banks is a teacher. They both grew up farming and gardening.
　　Sean Conroe and Amber Banks wanted to start a farm in the middle of Seattle. (20) They created a website to get volunteers and donations. Within a week, they were offered a plot of land between two houses. Sean Conroe says 20 volunteers worked for 6 weekends to turn the grassy land into a farm. All kinds of vegetables are grown on the farm, such as carrots, green onions, peas and beans. They have harvested about 90 kg of produce so far. They have donated most of it to local food banks that feed hungry people in Seattle. (21) Sean Conroe says their goal is to show city people the joy of growing food. He invites school groups to the farm to help out. And Amber Banks says they want the same people who get food donations to learn how to work the soil. Sean Conroe says their farm is expanding to other empty areas of Seattle. So are a lot of other urban farming groups. But the growth of these farms is limited. (22) That is because Seattle, like a lot of other cities, has restrictions on urban farms.
　　20. How did Sean Conroe and Amber Banks start their farm?
　　21. Why did Sean Conroe and Amber Banks build an urban farm?
　　22. Why is the development of the urban farms limited?