Cells are the most simple level of organization, as tissues contain cells, organs contain tissues and systems contain organs, so R would be your answer.
The correct answer is (c) discover antigens in body fluids.
B lymphocytes act as antigen presenting cells that recognizes and displays the antigen to the white blood cells. These cells process the antigens that are present in body fluids. Professional antigen presenting cells includes B-lymphocytes, macrophages, dendritic cells, etc.
Different enzymes have specific active sites which can only catalyze one <span>substrate. Enzyme-substrate complexes are produced only when the substrate fits into the active site :)</span>
The answer is <u>B.</u> <em>from carbon dioxide in the atmosphere</em>
