TOEFL Reading: ETS-TOEFL阅读机经 - 14I1H63RPP4JC9OEL$

According to paragraph 3, what is one possible explanation for why American workers change jobs more frequently than workers elsewhere do? A. The relatively small percentage of American workers who live in urban areas B. The tendency of American employers to provide less training for workers C. The recent decrease in mobility costs in the United States D. The lower average population density in the United States