[问答题]
A 21-speed bicycle has seven sprockets at the rear wheel and three a f u*4dsbc,31kjd/uvft the ped) p8qur nna65wyu-j .jal c56yn.rwajp j)q-8nuu ranks. 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?