Why is riding a bicycle on a well-maintained road more energy efficient than walking or running?

W) Bipedal motion is highly inefficient

X) The coefficient of sliding friction between tires and the ground is less than that between shoes and the ground

Y) Gravity does very little work on a bicyclist relative to a walker or runner

Z) Bicycling requires more calories than walking
Y) GRAVITY DOES VERY LITTLE WORK ON A BICYCLIST RELATIVE TO A WALKER OR RUNNER