The phospholipid bilayer of membranes allows lipids and small nonpolar molecules to pass across membranes,but prohibits the passage of ions and polar molecules.This occurs because the phospholipid tails are
A) always saturated.
B) hydrophilic and hydrophobic.
C) always unsaturated.
D) hydrophilic and selectively permeable.
E) hydrophobic and selectively permeable.
