Answer
d. All physical conditions except for one are identical for all groups tested.
Work Step by Step
During experiments, researchers aim to study the effects of a single variable. They do this by conducting an experiment multiple times with changes to only one variable and recording the results. By keeping the experimental and control group conditions the same, aside from the variable they are studying, they can determine if differences in the studied variable cause differences in results. A valid experiment must have consistent conditions.
The correct answer is (d).
