What role do prostaglandins play in the function of precapillary sphincters?
A. Prostaglandins may cause precapillary sphincters to open or close depending on the tissue.
B. Prostaglandins always cause precapillary sphincters to close.
C. Prostaglandins do not directly affect precapillary sphincters.
D. Prostaglandins always cause precapillary sphincters to open.