Final answer:
The horizontal motion in projectile motion with no air drag is independent of the vertical motion. This illustrates that the motions are separate and the vertical descent happens at the same rate regardless of the horizontal movement. so, option B is the correct answer.
Step-by-step explanation:
The correct idea about projectile motion with no air drag is that the horizontal motion is independent of the vertical motion. This principle is a fundamental aspect of classical mechanics and indicates that in an ideal situation without air resistance, the two motions do not influence each other. When a projectile is launched, its vertical descent due to gravity happens at the same rate whether it is moving horizontally or not.
For an object in projectile motion, assuming no air resistance, the only force acting on the object in the vertical direction is gravity, which causes a consistent acceleration downwards. In contrast, in the horizontal direction, after the object is projected, there are no additional forces acting on it, so it maintains a constant horizontal velocity. The paths of a projectile dropped from rest and one that is launched horizontally will intersect the ground at the same time if dropped from the same height, illustrating this independence of motions.