According to the "fluid mosaic model" of cell membranes, which of the following statement is true about phospholipids?
a) They have hydrophilic heads and hydrophobic tails.
b) They frequently flip-flop from one side of the membrane to the other side.
c) They occur in an uninterrupted bilayer, with membrane proteins restricted to the surface of the membrane.
d) They have hydrophilic tails in the interior of the membrane.