Most progress against air-pollution has been cheaper than expected. Smog controls on automobiles, for example, were ＿＿（47）＿＿ to cost thousands of dollars for each vehicle. Today's new cars emit less than 2 percent as much smog-forming pollution as the cars of 1970, and the cars are still as affordable today as they were then. Acid-rain control has cost about 10 percent of what was expected in 1990, when Congress ＿＿（48）＿＿ new rules. At that time, opponents said the regulations would cause a "clean-air recession"; ＿＿（49）＿＿ , the economy boomed. Greenhouse gases, being global, are the biggest air-pollution problem the world ever ＿＿（50）＿＿ .