Explain how the heart and the lungs work together to supply the muscles with oxygen.
! i do not get it!

The primary role of the heart is to pump oxygenated blood around the body to the working muscles and organs. In order for this to occur, deoxygenated blood from the heart must pass through the lungs before it goes out tot eh rest of the body. Deoxygenated blood that has entered the heart is then immediately pumped into the lungs to become oxygenated, through diffusion of CO2 and O2 in the capillaries on the lung surface (sacs called alveoli). Once the O2 is picked up by the blood cells, it is then free to pass on throughout the body through the blood stream, to the working muscles. Here, oxygen will be exchanged for CO2 once again (this blood is now DEoxygenated), and will then return to the heart, and the process will start again. 

The lungs obtain oxygen when it passes through the bronchioles, which are small "passageways" in your lungs. Once this oxygen is obtained, it is delivered to the blood. The heart then pumps that blood around your body to circulate it. The blood is delivered to the capillaries in muscles. All of the unnecessary gasses are then expelled from your lungs. This process then repeats itself.

Aand b would both be correct