[问答题]
A 21-speed bicycle has seven sprockets at-02funlx wa 7f the rear wheel and tham 3mp,7zfu(zk,q 3yfree at the pedal cranks. In whichpu3f, y qz(mzamk73 ,f 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?