The BBB Functions to Prevent Unwanted Molecules from Entering the Brain

The BBB functions to prevent unwanted molecules from entering the brain tissues, by which of these mechanisms?

A) requiring all molecules that pass through to use either a carrier or transport protein
B) selectively allowing some water-soluble molecules pass through with the help of proteins
C) selectively allowing some lipid-soluble molecules pass through with the help of proteins
D) requiring lipid-soluble molecules to use either a carrier or transport protein