TOEFL Reading: ETS-TOEFL阅读机经 - M3F575327473S133L$

What is the author's purpose in discussing "riding a bicycle?" A. To explain why walking requires more energy than swimming and flying B. To contrast the amount of energy used in riding a bicycle with the amount of energy used in swimming and flying C. To suggest that humans should ride a bicycle instead of running or walking in order to conserve energy D. To provide an example of an activity for which accelerations and decelerations require a great deal of muscular effort