[问答题]
A 21-speed bicycle has seven sprockets at the rear wheel andw1: x 25xsldvd dtf7(v t * c8s7.bqc-cxaryvo6 hree at the pedal cranks. In which gear is it harder to pedal, a small rear sprocket or a large rear sprocket? Why? In which gear is it harder to pedal, a small front sprocket or a large front sprxc.*cbq6y78 v- oacr socket? Why?