The correct answer is C.
Internal migration in China is one of the most extensive in the world. Between 1990 and 2015 the proportion of China's population living in urban areas jumped from 26% to 56%. There are estimated more than 200 million rural migrants working in big cities.
The main reason for rural to urban migration is the search for a job. That's why this type of migration experienced in China today is mostly temporary. Migrants work in the city to save money to be able to return to their rural hometowns later. Two-thirds of the migrants are male, one-third female.
Other factors that influence China's urban migration are : education, business opportunities and higher standard of living.
One of the biggest problems the migrants face is the applied system of household registration - hukou. Chinese people can only receive state welfare in their place of birth. Changing their place of living permanently would mean losing their land rights in their hometowns. This is the reason why many migrants feel obliged to return. The hukou rule works across generations, meaning that if a child is born in their parents' new residence, it will still get the hukou only in their parents' birthplace. This created a generation of 'homeless' children in China, the kids don't believe they belong anywhere, neither to the city they were born in nor their parents' hometown.