Answer:
because a complete equation must have all the elements present on the left hand side of the arrow on the right hand side of the arrow as well, and to be balanced there must be the same number of each type of atom, so both A and C are full equations, but A is not balanced because although there are two oxygen atoms on each side of the arrow, there is only one hydrogen atom on the left and two on the right, thus the right hand side is 'heavier' than the left and the equation is unbalanced.
Explanation:
