When blood from the heart enters the capillaries surrounding muscles in the legs, which of following take place? A) Increased CO2 concentrations in blood cause CO2 to enter muscles.
B) Increased CO2 concentrations cause oxygen to enter the blood.
C) Diffusion causes oxygen molecules to enter muscle cells.
D) Diffusion of oxygen molecules occurs in and out of the muscle cells.

is D

Explanation:

the answer is "a.) well–developed vascular system."

explanation:

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