DrFriesen Wants to Investigate Whether Store Clerks Behave in a Discriminatory

Dr.Friesen wants to investigate whether store clerks behave in a discriminatory manner toward teenagers.He is trying to decide between using naturalistic observation and using a questionnaire.What would be the best choice

A)  Questionnaires, because clerks would become angry and refuse to participate if they knew you were spying on them. 
B)  Naturalistic observation, because you can record behaviour and avoid self-report bias. 
C)  Questionnaires, because you'll be able to make cause-and-effect statements. 
D)  Naturalistic observation, because clerks would likely refuse to respond to questionnaires.