42.what is the source of the electric current produced by a voltaic cell?
a. a spontaneous oxidation-reduction reaction
b. competing salt bridges
c. non-spontaneous reduction at the cathode
d. an imbalance of ion charges

This is a spontaneos oxidation-reduction reaction. A would be the answer.

Explanation:

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.