In a study on the effect of calcium on bone density levels, 50 women were randomly chosen to receive calcium and 50 were chosen to receive a placebo. These subjects were later examined to check for differences in their bone density levels. In a second study on the effect of a new combination drug of calcium and vitamin D on bone density, 100 subjects were randomly chosen to receive the new medication and 100 were chosen to receive a placebo. The subjects were later examined to check for differences in their bone density levels. Which of the following statements is correct?

Group of answer choices

a. Both studies were observational studies.

b. Both studies were controlled experiments.

c. The first study was a controlled experiment, while the second was an observational study.

d. The first study was an observational study, while the second was controlled experiment.