Suppose a scientist wants to test the effects of a pesticide on mice, so he tags nearby families of wild mice. In each nest, he assigns some mice to the control group and some to the test group. He takes pictures of the mice and measures them. Then, he gives small doses of the pesticides to the mice in the test group. He returns regularly to make observations.

What is a probable source of error for this experimental design?
A.
The scientist could not control the dose given to the wild mice.
B.
The scientist's results were biased by his expectations.
C.
The scientist could not control the environment of the wild mice.
D.
The scientist did not use a control group in the experiment.