Which of the following would be the best way to improve the external validity of an experiment about whether or not people are more likely to help a disabled person open a door when they are in a good...

Which of the following would be the best way to improve the external validity of an experiment about whether or not people are more likely to help a disabled person open a door when they are in a good mood?

a. Make sure the experiment uses the observational method.

 b. Make sure it uses the correlational method to measure if mood correlates with helping behavior.

c. Make the experiment as realistic as possible and conduct it in a real-life setting instead of in a lab.

 d. Give the participants a good cover story