[问答题]
A 21-speed bicycle hahd7c gs 0cg73l2pd8zj ck- x8ms seven sprockets at the rear wheel and three at the pedal cranks. In whichd m8-8xpc2jzk 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?