[问答题]
A 21-speed bicycle has sevens.1rosfbx+.:h i 2obz sprockezp)sjm- .wy4zts at the rear wheel and three 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 spr)-zw.zjps4y m ocket? Why?