John Conducted an Experiment on Whether Listening to Different Types

John conducted an experiment on whether listening to different types of music affects people's stress levels.When participants arrived at the lab,they could choose to listen to classical music,folk music,or rock music.After listening for 15 minutes,John assessed how stressed his participants were by testing their saliva for stress hormones.The major mistake that John made with his research design was

A) He did not calculate the p-value.
B) He used an observational method when he should have used a correlational method.
C) He did not randomly assign his participants to conditions.
D) He should have used a field study instead of a lab experiment.
E) His independent variable is too complex.