[问答题]
A 21-speed bicycle haa-g, g-we uxm-s seven sprockets at the rearwe2c( 2z axhi3 wheel and three at the pedal crehx3i2wzc2 (aanks. 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 front sprocket? Why?