Answer:
d. act as a control that would ensure that the results obtained are due to a difference in only one variable.
Explanation:
The primary reason for including a comparison population within the design of this experiment in which there was a need to also examine a comparison population of humans in which births took place every month would be to act as a control that would ensure that the results obtained are due to a difference in only one variable. This would ensure more accuracy of the experiment in relation to the factor considered.