[问答题]
A 21-speed bicycle h*8pytd3u jbm7e :,hazas seven sprockets at thq*/ l; (aw32gofzw nlne rear wheel and three at the pedal cranks. In which g a w;z/lnnfq(3*g2l owear 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?