A farmer wishes to determine which of two brands of baby-pig pellets, Kent or Moormans, produces better weight gains. Two of his sows each give birth to litters of 10 pigs on the same day, so he decides to give the baby-pigs in litter A only Kent pellets, while the pigs in litter B will get only Moormans pellets. After four weeks, the average weight gain for pigs in litter A is greater than the average weight gain for pigs in litter B. The brand of pellets in this story is:

a. a parameter
b. the response variable
c. the explanatory variable
d. the placebo effect
e. stratified