3 The plasma membrane is semipermeable because -
A phospholipids let through both polar and nonpolar substances.
B proteins in the membrane act like gates to control entrance and exit of molecules.
C cholesterol makes the membrane too rigid to let small molecules through.
D moving materials through the membrane always requires the use of ATP.

B) proteins in the membrane act like gates to control entrance and exit of molecules.

Explanation: Plasma membrane is composed of phospholipids bilayer in which the hydrophobic regions of the phospholipids face each other to form the core of the bilayer. Proteins are embedded in the bilayer through an interaction between the amino acid side chains and the nonpolar acyl chains of the phospholipids. The embedded membrane proteins form channels for passage of ions and other molecules. Each membrane protein is selected for one or more ions or molecules that passes through it. The membrane is said to be semipermeable because it allows the passage of some molecules and excludes the passage of others.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."