Why does the level of carbon dioxide need to stay low in the plasma returning to the lungs?
a. in order for carbon dioxide to remain in the tissues
b. in order for oxygen to remain in the blood
c. in order for carbon dioxide to enter the blood
d. in order for hemoglobin to maintain its shape

B. In order for oxygen to remain in the blood

Explanation:

The hemoglobin is a protein in the blood whose function is to capture the oxygen in the lungs and carry it to the tissues and organs. This protein binds to oxygen but also, to carbon dioxide.

When the plasma in returning to the lungs the carbon dioxide needs to stay low because carbon dioxide has approximately 210 more affinity to hemoglobin than oxygen. This means that a single carbon dioxide molecule binds 210 times harder to the hemoglobin than oxygen. If the carbon dioxide levels are high the hemoglobin will bind to this CO2 instead of binding to the oxygen. If the hemoglobin is not carrying oxygen to the tissues and organs, they will suffocate due to a lack of oxygen and excess of CO2.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.