To study whether vitamin C is effective in shortening the duration of a common cold, a random sample of 300 volunteers with colds is selected, and the 300 volunteers are randomly divided into three groups of size 100. One group gets a supply of 1000 mg vitamin C pills and is instructed to take a pill each day until their cold symptoms vanish, a second group receives a supply of 500 mg vitamin C pills to be taken each day until their cold symptoms vanish, and the third group receives a supply of placebo pills to be taken each day until their cold symptoms vanish. The subjects are asked to record the duration of each cold they have in a one year period.

(a) What is the target population in this study?

(b) Is this an experiment or an observational study?

(c) What are the treatments in this study?

(d) Could this study be run as a double-blind study? Explain.