A Scientist Performs a Controlled Experiment.This Means That

A scientist performs a controlled experiment.This means that

A) two experiments are conducted,one differing from the other by only a single variable.
B) one experiment is performed,but the scientist controls the variables.
C) the experiment proceeds at a slow pace to guarantee that the scientist can carefully observe all reactions and process all experimental data.
D) the experiment is repeated many times to ensure that the results are accurate.