In one study subjects were randomly given either 500 or 1000 milligrams of vitamin C daily, and the number of colds they came down with during a winter season was noted. In a second study people responded to a questionnaire asking about the average number of hours they sleep per night and the number of colds they came down with during a winter season.

Select one:

a.

The first study was an experiment without a control group, while the second was an observational study.

b.

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

c.

Both studies were controlled experiments.

d.

Both studies were observational studies.

e.

Each study was part controlled experiment and part observational study.