A cell membrane consists of a lipid bilayer, which is a double layer of phospholipids. Each phospholipid has a hydrophilic phosphate head and a
hydrophobic lipid tail. These phospholipids are not chemically bound to
one another, but rather maintain their structure due to the fact that the
lipid tails do not like water. They're hydrophobic. If they all buddy up, then
they can all stay dry. (See the figure below.) What is the function of the
phospholipid bilayer ?

Amath expression is a collection of one or more terms, which are separated by the signs ( + , - , x , / ).example :   8+b8+b  or  a−3a−3in algebra, an equation is a mathematical statement with two expressions where, each expression is separated by an equal (=) sign. variables can be present on both the sides of the equality (=) sign and we use letters to denote such variables.example 1:   x=4+2x=4+2  , [here, we can observe that the variable is present only on one side of the equation].example 2:   2x+3=x+42x+3=x+4  , [here, we can observe that the variables are present on both side of the equation].

I'm pretty sure it's c. extremely gradual but not 100% sure though.