Answer:
b., four; four
Explanation:
As an element, carbon is tetravalent, it has 4 outer shell electrons and oxygen has 6 outer shell electrons. The carbon dioxide molecule is formed from one carbon atom and two oxygen atoms by two double covalent bonds. In making CO₂, two electrons of carbon share with each oxygen atoms resulting in 4 bonding electrons in total. Carbon dioxide's strong double bonds make it very stable.
In reaction with water, carbon forms double covalent bond with one of the oxygen of CO₂ and with two single covalent bonds one with other oxygen atoms of CO₂ and one with the oxygen atom of H₂O forming carbonic acid (H₂CO₃). Thus, carbon shares 4 electrons.
