How did mass immigration and migration accelerate urbanization

There were mass immigration and migration in second half of the 19th century. There were two kind of factors which forced people to migrate : 1st push factors- like famines, religious and political persecution etc and 2nd were pull factors-like employment and religious and political freedom.

People migrated in large numbers to urban areas to find an employment in factories and industries.These people coming from different areas and countries began to live near these factories and industries. It resulted in building infrastructure and more factories to meet the product demand of these workers.This in turn created more jobs and more building of houses near these cities. In this way mass immigration and migration helped to accelerate urbanization.

the correct answer is b, "fertility rates are increasing, so populations will grow more quickly in the future."