Read the passage and circle the letter A, B, C, or D to indicate the best answer to each of the following questions.

Nowadays big cities are becoming more and more populated because of people's tendency of living in big cities. Even though it has many advantages like job opportunities, higher living standard and modern lifestyles, living in a big city may have many drawbacks.

Urban centers are filled with motorized transport and other sources of pollutants like waste from a large number of people and factories in the suburbs. The urban centers have very few or even totally missing green spaces with higher amounts of pollution. Gradually, the citizens who have experienced pollution every day suffer from various diseases like lung cancers, asthma and bronchitis.

Another disadvantage of living in a city is traffic congestion. Every day, many people have to struggle with traffic jams, so they are sometimes late for work or stuck on the roads for over fifteen minutes. In addition, too many vehicles on the street during rush hours are one of the causes of traffic accidents.

It is undeniable that the cost of living in big cities is much higher than that in the countryside. The impact of this can result in the high costs of the public services, such as healthcare, food and education. It is difficult for the poor and workers to live in big cities with low wages. They seem to struggle with the fast changes in the prices of basic necessities and public services like hospitals and schools.

In conclusion, the more people live in a city, the more social issues they have to deal with. It is urgent that local governments find effective solutions to make sure a healthy living environment for their citizens.

What does the passage mainly discuss?