In Transmission EM Preparations of Cells the Cell Membrane Often

In transmission EM preparations of cells the cell membrane often appears as a trilaminar structure having two parallel dark-staining components on either side of an unstained middle layer. This central poorly stained region of the membrane is primarily responsible for which of the following functions?

A) Creation of a barrier to water-soluble molecules
B) Binding by cellular receptions to specific ligands
C) Catalyzing membrane-associated activities
D) Transport of ions
E) Connections to the cytoskeleton