Why is the plasma membrane called selectively permeable membrane ?

Plasma membrane is a lipoproteinous and trilaminar membrane. It regulates the exchange of materials between the cystosol and extra-cellular medium. Give the reasons for the following: ( C ) Why the plasma membrane is said a selectively permeable membrane?

Why is plasma membrane called a semipermeable membrane? Give its significance.

Why is plasma membrane called quasi-fluid membrane?

What is a semi-permeable membrane ?