[问答题]
A 21-speed bicycle has sevedzgo .:3u0/ob h q4,he; xgylin sprockets u cz(ws .v6cb(;jxv5zat 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,bc .(v6(w5j zv s;xzuc a small front sprocket or a large front sprocket? Why?