When Membrane-Bound Receptors Like HER2 Bind to Molecules Outside of a Human

When membrane-bound receptors like HER2 bind to molecules outside of a human cell,they must move laterally within the bilayer and also bind to a second HER2 receptor,forming a dimer (2-part molecule) ,in order for the signal to be further tranferred inside the cell through the membrane.This is possible because the cell membrane is

A) a fluid mosaic.
B) hydrophobic.
C) a selectively permeable barrier.
D) rigid like a cell wall.
E) both selectively permeable and a fluid mosaic.