PVC Propeller VS Metal Propeller

PVC is:

Why there two propellers in a helicopter ?

Assertion : There are two propellers in a helicopter. Reason : Angular momentum is conserved.

A alt of cellulose used in propellent explosives is

A combination of nitrocellulose and nitroglycerine is a propellant of which of the following types?

An airplane engine starts from rest, and 2 s later, it is rotating with an angular speed of 300 rev/min. If the angular acceleration is constant, how many revolutions does the propeller undergo during this time?