Two projectiles are in flight at the same time. The acceleration of one relative to the other: A. is always 9.8 m/s2 B. can be as large as 19.8 m/s2 C. can be horizontal D. is zero E. none of these

D. Is Zero

Explanation:

The acceleration on projectiles travelling through the air is only caused by gravity. This equal 9.81 m/s^2.

Other than gravity, there is no source of acceleration. Only air resistance causes projectiles of different surface areas to slow down at different rates.

As friction is not considered here (because no statement about surface areas of projectiles is given in the question), we do not need to consider it's effects and thus ONLY GRAVITY MATTERS.

Since gravity acting on both objects is equal, they have no acceleration relative to one another.

scientific laws are the result of testing theories and proving them to be facts.