Answer:
C. the effects of an earthquake on blood pressure
Explanation:
Natural experiments are observational studies in which participants are randomly assigned to groups that allow the researcher to draw causal relationships between an event and a factor. These experiments are used in studies where it would be unethical and unacceptable to subject human participants to such conditions. It is most times applied in epidemiological studies.
Since the researcher cannot control an earthquake which is a natural occurrence, this can be rightly described as a natural experiment. Controls are not used in this type of experiment.
