With more and more farmers working in big cities, their children go into the cities, too.However, problems appear. One of these problems is that is difficult for these children to receive education in cities. Luckily, there are some free schools for them, but they are not enough. Ten-year-old Li Xin is one of the lucky girls who get a chance to study in one of these schools. She is very happy. She studies in a school specially for migrant workers' children. For children like Li Xin who don't have enough money, these free schools are their best choice, because they have free classes, textbooks, uniforms and meals in the schools. But there is a long way to go. The free schools for children from migrant workers' families are not enough because to _ such a school is not easy and needs lots of money. A school officer said they needed at least one million yuan to run the school. "Luckily, some people and companies donate money, used books and a lot of other things to help us. There are also some volunteers teachers," he added. Anyhow, we must try to give these children an equal chance to get better education so that we can make our society harmonious . The passage mainly talks about  _  .
what jobs migrant workers can do in big cities the development of big cities helping migrant workers' children go to school what difficulties those free schools face now