He tests multiple hypotheses at the same time. This would introduce the possibility for a confounding variable to alter his results.
Answer:
The correct answer is by testing multiple hypothesis at a time and by using different kinds of soil in each of the samples.
Explanation:
An external effect that changes the effect of an independent variable and the dependent variable is considered as a confounding variable. This extraneous effect has an application in influencing the result of experimental design. The confounding variables can hamper and can produce unnecessary results in an experiment.
In the given case, by testing multiple hypotheses at a time and by using different kinds of soil in each of the samples the confounding variables can be presented.
