Please help this is on my exam

D. The time of day

Explanation:

The independent variable is the variable the experimenter changes or controls and is assumed to have a direct effect on the dependent variable

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

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