[问答题]
A 21-speed bicycle has seven sppatnmto7f+ez xe8-3 4rockets at the rear whee 6vy;w hyipa(+l and three at the pedal cranks. In which gea wy6y(;vha+pi r 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?