Donnie Wants to Know Whether Attractive Waiters Make More Tips

Donnie wants to know whether attractive waiters make more tips.He has a group of people rate the attractiveness of five different waiters,and he gets the waiters to tell him how much money they make in tips every night for a month.What type of research design has Donnie used

A)  quasi-experimental design 
B)  correlational design 
C)  experimental design 
D)  case study design