Which system monitors carbon dioxide levels in
the blood?

Heart control level of oxygen and carbon dioxide

oxymeter measures level of oxygen...

There are two kinds of respiratory chemoreceptors: arterial chemoreceptors, which monitor and respond to changes in the partial pressure of oxygen and carbon dioxide in the arterial blood, and central chemoreceptors in the brain. hope this helps you

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in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation:

i think it's c. if you look search electronic symbol on google and then click on wiki it shows you a