Brian Was of Average Weight Throughout Childhood and in College

Brian was of average weight throughout childhood and in college.However,he has gained more than 30 pounds after he started working,and is now finding it very difficult to lose the excess weight and to maintain weight loss.His doctor tells him that one of the reasons for this may be because:

A) the set point he was born with cannot be changed.
B) he may not be able to get rid of the fat cells he gained when he put on weight.
C) there is a lower availability of energy-saving devices.
D) the human genome has changed markedly in the last century.