Which of the Following Is True of an Experiment

Which of the following is true of an experiment?

A) It includes the voluntary participation of respondents in surveys, allowing individual marketers to add a small number of questions to an existing survey to receive cost-effective data on their questions.
B) It can be obtained by watching how people behave, either in person or by using a machine to record events.
C) It is a hybrid of primary and secondary research conducted by a research company, spreading the cost across many clients to reduce the price.
D) It involves changing a variable involved in a purchase to find out what happens.