If You Were to Design a Long-Term Research Study to Determine

If you were to design a long-term research study to determine why there are no human births in Lapland during the months of August, September, and October, you would need to also examine a comparison population of humans in which births took place every month. The primary reason for including a comparison population within the design of this experiment would be to:

A) accumulate more facts that could be reported to other scientists.
B) test the effects of more than one variable at the same time.
C) prove that there are no births in Lapland during August, September, and October.
D) act as a control that would ensure that the results obtained are due to a difference in only one variable.