Cocoa plant type 1 yielded 155 bushels per acre last year at a research farm. This year, Cocoa plant type 2,
planted in the same location, yielded only 123 bushels per acre and there was a major freeze. The researchers do not know whether the difference is the result of the superiority of type 1 or the freeze. What is this an example of?
b.) confounding variable
c.) matched pairs design
d.) the placebo effect