I think b. Proteins are polymers of amino acids is the correct answer
excretory system: removes waste from blood.
respiratory system: obtains oxygen so the energy in the nutrients can be extracted.
circulatory system: transports nutrients from small intestine to the othet tissues.
nervous system: regulates and controls digestive functioning.
( Am not completly sure, but i think this is the answer)
They made the cell theory, concluding that all living things are made of cells.
