Answer the following questions related to the motion of a projectile projected from ground level and landing back at the same level.
a. (i) Define a projectile and state the principle of independence of horizontal and vertical motion for a projectile.
b. (ii) Explain qualitatively why the horizontal component of velocity of the projectile remains constant throughout its flight while the vertical component continuously changes.
c. (iii) Name the physical quantity responsible for the change in the vertical component and specify its direction.