Read the passage and choose the best answer to each of the questions below. Large cities often have problems that small towns and rural areas do not have. Two of the biggest problems are heavy traffic and the pollution that cars create. Of course. Traffic problems and pollution are not only found in big cities. However, the higher populations and larger number of cars on the roads in cities can make the problems happen more often and with more noticeable effects. One of the most common traffic problems the large cities have is congestion. As the population of a city increases, so does the number of cars on the road. Cities cannot always improve the number and size of their roads and highways to keep up with the number of cars. The result is traffic congestion, or traffic jams. When traffic jams happen, cars that are stuck in the congestion continue to run their engines. This creates pollution and is a big problems. Pollution causes health problems for the people in cities and also hurts the environment. The high number of cars is caused by __________.   A.the population B.urban planning C.traffic congestion D.environmental pollution