In the Fluid Mosaic Model of Cell Membranes, What Does

In the fluid mosaic model of cell membranes, what does the "fluid" part of the model refer to?

A) the constant movement of the hydrophilic tails in the interior of the membrane
B) the phospholipid molecules, which freely move and exchange places with other lipids within the same half of the bilayer
C) the free movement of cholesterol molecules within the membrane
D) a thin layer of water found sandwiched between the two layers of phospholipids