A Scientist Performs a Controlled Experiment

A scientist performs a controlled experiment. This means that

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) two versions of the experiment are conducted, one differing from the other by only a single variable.
D) one experiment is performed, but the scientist controls the variables.