Apharmaceutical company manufactures medicine to reduce pain caused by migraine headaches. the company is investigating whether a new medicine is more effective in reducing pain than the current medicine. a random sample of 500 participants who experience migraines was selected, and the participants were randomly assigned to one of two groups of equal size. the first group received the current medicine and the second group received the new medicine. when a participant experienced a migraine, he or she was instructed to take the medicine and, 15 minutes after taking the medicine, to rate the pain relief on a scale from 1 to 10, with 1 being no relief to 10 being complete relief. at the end of six months, the average pain relief for each participant was calculated. which of the following is the best description of the study?
a. an experiment using a completely randomized design
b. an experiment using a matched-pairs design
c. an observational study using a simple random sample
d. an observational study using a cluster sample
e. an observational study using a stratified sample