How does the law of conservation of mass apply to this reaction: C2H4 + 3O2 → 2H2O + 2CO2?

A. Each element needs to be balanced.

B. The law of conservation of mass has already been applied. There is an equal number of each element on both sides of the equation.

C. Only the oxygen needs to be balanced. There are equal numbers of hydrogen and carbon.

D. The equation needs to be balanced. There are fewer oxygen atoms in the equation than hydrogen or carbon.