Solve the all parts of given problem:
Problem: A projectile is fired from horizontally from a gun that is 45m above flat ground; emerging from the gun with a speed of 250 m/s.
Part A: How long does the projectile remain in the air?
Part B: At what horizontal distance from the firing point does it strike the ground?
Part C: What is the magnitude of the vertical component of its velocity as it strikes the ground?
Express your knowledge to solve the above problem.