Some animals have no gills when young, but then develop gills that grow larger as the animal grows larger. Which of the following statements provides the best explanation for this increase in gill size?

A) The young of these animals are much more active than the adult, which leads to a higher BMR (basal metabolic rate) and, therefore, a higher need for oxygen.
B) Relative to their volume, the young have more surface area across which they can transport all the oxygen they need.
C) The young have a higher heart rate and thus a lower demand for oxygen.
D) Relative to their surface area, the young have more body volume in which they can store oxygen for long periods of time.