Passage 2In the past two years, millions of Americans have suddenly taken an interest in the bicycle as if it were a startling new invention. Annual bike sales doubled between 1960 and 1970, and there are nearly 70 million bikes in the United States today. That’s more than two for every three automobiles.Of course, the bike has been around for more than 150 years, and this isn’t America’s first bicycle boom(兴旺). A wave of bike enthusiasm swept the land in the late 1800s and bicycle production hit two million units in 1897. Then with the coming of the auto, bicycling declined, and for decades remained popular only with children and a few adults.Now, national concern with air pollution and physical fitness has brought the bike back to the forefront ― particularly with adults. More than eight million bikes were sold in the United States last year and a third of them went to adults. The year before, only 15 percent of new bike sales were for adults.36. In the United States, the bicycle is______.