What needs to happen for the equation CH4 + O2 → CO2 + H2O to be balanced?          
A. Nothing, the equation is already balanced.    
B. A coefficient of 2 must be added before the second reactant and the second product.   
 C. A coefficient of 2 must be added before both products and reactants.   
 D. A coefficient of 2 must be added before both products.