A、They realize the importance of keeping fit. B、There are too many traffic jams. C、The economy is slowing down. D、Gas is in shor

More and more Americans have been leaving their cars at home and riding to work on bicycles. They are doing this because of gas prices, a slowing economy and concerns about the environment.
    Andy Clark is the executive director of the League of American Bicyclists. His group supports bicycling for fun, fitness and transportation. Clark says this is good news for the environment. He says riding a bicycle to work does not burn fossil fuel or create dangerous pollutants. Experts say the effects are the most important on short trips. Shorter car trips release more pollution into the air for each kilometer driven. This is because the device in a car engine that reduces the harmfulness of emissions needs to warm up before it can work well.
    Last year, the Pacific Northwest city of Portland, Oregon, had the highest percentage of bicycle commuters in the United States. Portland has been doing progressive city planning for many years to create special paths for bike riders. Andrew Land is one of Portland’s citizens who bikes to work every day. Mr. Land is 33 years old and has never owned a car. He has biked to work for 12 years. Before moving to Portland 6 years ago, he lived in Washington, D.C. But he was hit by a car twice while biking to work there. That has not happened in Portland where there are special roads for bicycles.
20. Why do more and more Americans choose to ride to work?
21. What’s the purpose of the progressive city planning in Portland?
22. What do we learn about Andrew Land from the passage?