A control group is the version of the experiment that does not get the treatment, so you have something to compare against. If every plant gets fertilizer, you cannot tell what the fertilizer did. Experimental design is in experimental design.
What stays the same
The control still gets the same light, soil, and time. Only the independent variable is withheld. Independent versus dependent is in independent and dependent variables.
Two experiments that differ by one factor are a comparison, in comparing two experiments. Hypothesis versus what the data actually show is in hypothesis versus conclusion.
The bottom line
The control is the baseline. If the passage never ran one, you cannot claim the treatment caused the change unless the question gives you another comparison.
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