In an appropriately designed experiment, a scientist is able to test the effect of:
a) a single variable
b) multiple variables
c) the hypothesis
d) scientific observations

In an appropriately designed experiment, a scientist is able to test the effect of A. a single variable.

In an experiment that's appropriately designed, there is typically one independent variable. The independent variable is the variable that one manipulates in an experiment.

In an appropriately designed experiment, a scientist is able to test the effect of a single variable. This allows the scientist to determine if the variable that is manipulated has an impact on the subject.

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A) a single variable

Explanation:

An appropriately designed experiment usually involves only one independent variable. This allows the scientist to truly determine whether the variable manipulated is what had an effect on the subject. You may test more than one variable, but this is usually done or experimented separately, or one at a time.

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