Controlled experiments are experiments that have only one variable. All other factors in the experiment are kept the same. Many hypotheses can be tested through controlled experiments, but it is not always possible to do a controlled experiment.

Which of the following hypotheses most likely cannot be tested with a controlled experiment?
A.
Ultraviolet radiation can cause genetic mutations in bacteria.
B.
Gravitational force is affected by both mass and distance.
C.
A flowering plant will produce more fruit when treated with phosphate powder.
D.
In the wild, chimpanzees form social groups composed of 10 to 20 individuals.