The independent variable:
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a. is the one that the experimenter manipulates directly.
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o
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b. tells you what happens in the experiment.
c. is not present in all experiments.
d. is what happens to the experimental group.

A. Is the one that the experimenter manipulates directly

Explanation:

The independent variable is the one that is manipulated during an experiment by the experimenter.

The dependent variable is the one that is effected by the independent variable in an experiment.

c because the action was the roller coaster reaches bottom of hill (action) reaction would be blood tends to drain away from head

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