An aerospace engineer is designing a rocket to have three stages. Which is not a reasonable explanation for this decision?
A. To simplify the design of the rocket
O B. To decrease the mass of the rocket as it flies
C. To take advantage of the lesser amount of thrust needed in space
D. So the rocket only carries necessary mass

A. To simplify the design of the rocket.

Explanation:

3 stage rockets require huge amounts of fuel when launched in order to last the entire voyage.

As the rocket flies, some compartments may be detached and fall away from the rocket, decreasing its mass (eliminating B, as it is reasonable). This is done so that the rocket only carries necessary mass, which doesn't include the used up rocket parts (eliminating D, as it is reasonable). When the rocket finally reaches space, the decrease in gravity means less thrust is needed, which is an important thing to take note of while designing the rocket (eliminating C, as it is reasonable).

A three stage rocket is very complex and has many features and mechanisms that are complicated to add and deal with. Giving a rocket, three stages is not a simplification, so A is the answer.