[问答题]
A 21-speed bicycle has seven sprockets at the re 7 ji-v;)ycn9hep- gkbar wheel and three at the .ex77 qfdy5jmpedal cranks. In which gear is it harder to pedal, a small rear sprocket or a lar 7x7eydjqf.m5 ge rear sprocket? Why? In which gear is it harder to pedal, a small front sprocket or a large front sprocket? Why?