Why Would a Transverse Diffusion or Flip-Flop of Membrane Phospholipids

Why would a transverse diffusion or flip-flop of membrane phospholipids from one leaflet to the other be so rare?

A) Flippases are enzymes which are transiently stable and expressed at low concentrations.
B) For flip-flop to occur, it would be necessary for the hydrophilic head of a phospholipid to move through the hydrophobic part of the bilayer, an extremely unlikely event.
C) For flip-flop to occur, it would be necessary for the hydrophobic head of a phospholipid to move through the hydrophilic part of the bilayer, an extremely unlikely event.
D) Other membrane activities like flexing and lateral shift inhibit phospholipid flip-flop.