Which statement best answers the question, "why is copper a good conductor of electricity? "?

a. copper atoms have a loosely held electron in their outer shell that is able to move freely to other atoms.
b. copper atoms are tightly bound and are able to work with other copper atoms to exchange electrical charges.
c. coppers has an equal number of protons and electrons making the flow of current possible.
d. copper's outer shell electrons are not balanced and must join with other copper atoms in order to be stable.