So great has been the population flow toward the city that entire hinterland villages stand vacant or nearly so. About 120,000 people from outlying provinces move to Athens every year, with the result that 40% of Greece's citizenry are now packed into the capital. The migrants come for the few available jobs, which are usually no better than the ones they fled. At the current rate of migration, Athens by the year 2000 will have a population of 6.5 million, more than half the nation.
Aside from overcrowding and poor public transport, the biggest problems confronting Athenians are noise and pollution. A government study concluded that Athens was the noisiest city in the world. Smog is almost at killing levels: 180,300mg of sulfur dioxide per cubic meter of air, or up to four times the level that the World Health Organization considers safe. Nearly half the pollution comes from cars. Despite high prices for vehicles and fuel ($2.95 per gallon), nearly 100,000 automobiles are sold in Greece each year; 3,000 driver's licenses are issued in Athens monthly.
After decades of neglect, Athens is at last getting some attention. In March a committee of representatives from all major public service ministries met to discuss a plan to unclog the city, make it livable and clean up its environment. A save-Athens ministry, which will soon begin functioning, will propose heavy taxes to discourage in-migration, a minimum of $5 billion in public spending for Athens alone, and other projects for the countryside to encourage residents to stay out. A master plan that will move many government offices to the city's fringes is already in the works. Meanwhile, more Greeks keep moving into Athens. With few parks and precious few oxygen-producing plants, the city and its citizens are literally suffocating.
According to the passage, one of the biggest problems confronting Athenians is noise, which is mainly caused by ______.

A. buildings being demolished
B. highways being repaired
C. music being played
D. car horns being used