[问答题]
A 21-speed bicycle has seven sprocksj-s p nz*y6l9ets 7p4tvq mkmd1:at the rear wheel and three at the pedal cra1:dqkt7v m4 pmnks. 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 sprocket? Why?