Answer the following question given this paragraph: 

Plasma membranes are primarily made up of phospholipids (orange). The hydrophilic ("water-loving") head and two hydrophobic ("water-hating") tails are shown. The phospholipids form a bilayer (two layers). The middle of the bilayer is an area without water. There can be water on either side of the bilayer. There are many proteins throughout the membrane.


Q: What substances that primarily comprise plasma membranes form a bilayer?

 Choices:

- amino acids

- enzymes

- steroids

- phospholipids

A:
phospholipids
