The answer is C. A and B are obviously out as the digestive system definitely do not increase the amount of Oxygen or increase the rate of delivery of blood. The confusion is between option C and D. Here's a tip to help you remember, when there's 2 variables, and both are very similar, at times you can confuse between both, so just remember ONE. this will help you remember the other as you know the one you remembered is definitely correct. Anyway, the respiratory system is the one that increases the supply of oxygen to the blood.