John Conducted an Experiment to Test the Effectiveness of a Smoking

Multiple Choice

John conducted an experiment to test the effectiveness of a smoking cessation program. The experiment took place over a 1-month time period. Participants in the control group and the experimental group (those who participated in the smoking cessation program) recorded the number of cigarettes that they smoked each day. John was unaware that "national stop-smoking week" also happened to take place during the 1-month time period of his experiment. John's experiment is now confounded by​

A) maturation.​
B) history.
C) regression to the mean.
D) mortality.