[问答题]
A 21-speed bicycle has lrgukw fr m50)g/ 00e.mwti,hgz nv1*bhll9)seven sprockets at the rear wheel and three 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 a large frolzh )v*l9gbn1 nt sprocket? Why?