A tea manufacturer is trying to decide whether to add peach flavoring for mango flavoring to green tea for a new product. A study is done by stopping customers at a grocery store and asking them to drink a small cup of tea with each flavoring, then to rate each of the two flavors on a scale from 1 to 10. The order in which the teas are presented is randomly assigned for each person. What type of study is this?
a. An observational study.
b. A sample survey.
c. A case-control study.
d. A matched-pairs design.