[问答题]
A 21-speed bicycle hn:l*:ps d g9kang8z 8 r20skztas seve ;w;zyz6 e3ubtn sprockets at the rear wheel and three at the pedal cranks. In which gear is it harder to pedal, a small rear sprocket u;t 3 ze;bwy6zor a large rear sprocket? Why? In which gear is it harder to pedal, a small front sprocket or a large front sprocket? Why?