Tokyo, the capital of Japan, is one of the largest cities in the world. It is also one of the world's most modern cities. Twice last century, the city was destroyed and rebuilt. In 1923 a major earthquake struck the city. Thousands of people were killed and millions were left homeless as buildings collapsed and fires broke out throughout Tokyo. It took seven years to rebuild the city. During World War II, Tokyo was destroyed once again. As a result of these disasters, there is nothing of old Tokyo remaining in the downtown area. After the war, the people of Tokyo began to rebuild their city. Buildings went up at a fantastic rate, and between1945 and 1960, the city's population more than doubled. Because of the Olympic Games held in Tokyo in 1964, many new stadiums, parks and hotels were built to treat visitors from all over the world. As a result of this rapid development, however, many problems have risen. Housing shortage, pollution, and waste disposal  have presented serious challenges to the city, but the government has begun several programs to answer them. Which of the following problems is NOT mentioned in the passage?
Housing shortage. Waste disposal. Heavy traffic. Pollution.