A controlled experiment is one in which
A) the experiment is repeated many times to ensure that the results are accurate.
B) the experiment proceeds at a slow pace to guarantee that the scientist can carefully observe all reactions and process all experimental data.
C) there are at least two groups, one of which does not receive the experimental treatment.
D) there are at least two groups, one differing from the other by two or more variables.
E) there is one group for which the scientist controls all variables.
Answer: C
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