Pablo and Johanna have to do a yearlong study for their biology course. After some discussion, they decide to try comparing their dogs and the diet that they feed them. Each has a dog from the pound, and both dogs are less than one year old. Pablo feeds his shepherd-mix dog a special diet of wet and dry foods from the local vet, while Johanna uses generic dry kibble from the supermarket for her bulldog. They want to see which diet results in bigger, healthier, faster-growing dogs. According to the information given, one dependent variable in this study will be .

a. the age of the dogs
b. the sex of the dogs
c. the type of food the dogs receive
d. how much the dogs grow
e. the breed of the dogs