[问答题]
A 21-speed bicycle has seven sprox* j70ugdfpn(6n/j pzckets at the rear wheel and -lji9 b*eh+o5. jacqethree 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 jbq9 e.j *-lahcoi5+e front sprocket? Why?