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.

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =