[问答题]
A 21-speed bicycle has laxe5.3 u)uwi seven sprockets at the rear wheel and zt 2amt2 8tgo*three at the pedal cranks. In which gear is it harde2toz8 atm2*gtr 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?