[问答题]
A 21-speed bicycle has seven sprockets at the rear when/tr4*.prnc gf9not6 el a6ne184svy sin nd three at the pedal cranks. In which 1 68syvsie4nngear 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?