Soy bean type 1 yielded 125 bushels per acre last year at a research farm. This year, soy bean type 2, planted in the same location, yielded only 100 bushels per acre and there was a drought. The researchers do not know whether the difference is a result of the superiority of type 1 soy beans or the drought. What is this an example of?

A - Bias

B - Matched-pairs design

C - The placebo effect

D - Confounding variable

E - Replication