Which of the Following Is an Example of a Natural

Multiple Choice

Which of the following is an example of a natural experiment?

A) A laboratory research on the effectiveness of solar power as an alternative source of fuel
B) A research on the effectiveness of a new medicine among some voluntary participants
C) A research on the effect of air pollution on lung disorders by observing the health conditions of people who stay close to industrial areas and those who stay away from industries
D) A study on the benefits of regular exercise by paying for the membership fees at fitness clubs for one-half of the participants