Which of the following best describes the collection of independent samples?

a. Thirty pairs of twins are selected for astudy. One sample consists of the measurements on one of thetwins and the other sample is the same measurement on the other twin in each pair.

b. Twenty individuals participate in a weight reduction study. The before weight for each participant is put intosample 1 and the after weight is putinto sample2.

c. Twenty married couples are selected and the number of years of education for the husband is recorded in sample 1 and the number of years of education for the wife is put into sample 2.

d. Twenty rats are randomly divided into two groups of ten each. One group is a control group and the other group is an experimental group.