A controlled experiment is one in which A. there are at least two groups, one of which does not receive the experimental treatment. B. the experiment is repeated many times to ensure that the results are accurate. C. there are at least two groups, one differing from the other by two or more variables. D. there is one group for which the scientist controls all variables.

A. there are at least two groups, one of which does not receive the experimental treatment.

Explanation:

A controlled experiment is used by researchers to test a single variable at a time. The controlled experiment usually comprise of two groups: one group is the experimental group which receives experimental treatment, while the second is the control group which does not receive experimental treatment and is used to compare the experimental group.

In other words, during investigation, the control group is not exposed to an independent variable unlike the experimental group, hence, the control group serves as the baseline to measure changes in the experimental group.

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