During respiration A.) carbon dioxide is used to break down glucose into energy
B.) oxygen is used to break down glucose into energy
C.) carbon dioxide and water are combined to make glucose
D.) oxygen and water are combined to make water

C6H12O6+6O2--->6CO2+6H2O+energy

Oxygen(O2) is used to break down glucose(C6H12O6) into energy so the answer is b

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products