[问答题]
A 21-speed bicycle has seven /ejjsq3l-wi: zk//m x sprockets at the rear wheel and three e0na :amerk; *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 :mraa 0ke*n; ea large front sprocket? Why?