The manufacturer of a new toothpaste claims that the new product helps whiten teeth. Sixty subjects were randomly selected and had the whitness of thier teeth measured during examination. Each subject used the new toothpast for four months. The whitness of their teeth was again measured, and the result of the two measurements were compared. Which of the following statements is true?
a) This is not an experiment since no control group was used.
b) This is an observational study using a completely randomized design.
c) This is an experiment using a completely randomized desigh.
d) This is an experiment using a matched pair design.