When is momentum conserved?

a. it is conserved when only two objects collide; more than two objects can't conserve momentum.

b. it is always conserved.

c. momentum is conserved when the system is closed, which means there are no forces, like friction, acting on it.

d. momentum is never conserved.

Which statement is true about an object that is moving in a circular motion due to centripetal force, f, when the radius of its circular motion is doubled? the new force then becomes equal to 2f the new force then becomes equal to f the new force then becomes equal to f/2 the new force then becomes equal to f2 the new force then becomes equal to 1/f