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Answer:
(3) a single variable
Explanation:
A control experiment can be defined as an experiment in which a condition assumed to be a probable cause of the effect is being compared to the same situation by the scientist without involving or using the suspected condition.
Controlled variable refers to anything or quantity such as group, person, event, etc., that is held constant by the researcher during an experiment and as such is limited.
The first trial of a controlled experiment allows a scientist to isolate and test a single variable.
Generally, most scientific experiments usually have a control group so as to avail scientists (experimentalists) an ability to compare the outcome of their test results before testing a single variable. This control group is not given the treatment or influenced by the same single (independent) variable as the experimental group.
Therefore, this is the reason why science completely or totally rejects any hypothesis which is not supported by observations, as well as the results obtained from control experiments.
This ultimately implies that, for any hypothesis to be acceptable in science, it must be supported by observations and the results of control experiments; this give rise to factual informations, theories and by extension solutions to problems.