Which statement about precapillary sphincters is CORRECT?
A. They are openings that allow large molecules and blood cells to leave the capillary.
B. They are voluntary.
C. They can close off the capillaries by contracting.
D. They contract when oxygen levels in the tissue are low.
E. When the precapillary sphincters are open, blood flows only through the thoroughfare channels.
Answer: C
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