Sodium is a metal m, like copper. suggest reasons why sodium cannot be used in electrical wires.

Sodium is a metal but we use copper for electrical wires not sodium because sodium is highly reactive among these metals and would react with other substances, so if we use sodium wiring it can cause many problems and obviously no one wants sodium wiring in their houses. sodium has only one valence electron and that causes sodium be to be unstable. 

Most likely a is the answer - because  you generate   38 atp's but also consume an atp.   each atp gives approx. less than 4 kcal so probably a is the closest answer.