[问答题]
A 21-speed bicycle has seven sud- cetjcp.:lk. /dj+procketf2p f;ds.lct (s 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 halcsff(p;t. 2 drder to pedal, a small front sprocket or a large front sprocket? Why?