If there is no dropped ceiling, or if the drop is not as much as the height of the recessed luminaires, you will find that the center-to-center spacing of recessed luminaires will have to be in increments of ft.

The answer is "4".

Explanation:

The luminaire would be recessed inside a wall, so that, dependent on the surface mountings, its top-level is flush with the ceiling. It is the hanging under the primary structural, in which the drop was an area of the above falling ceiling, that referred to its full space, because it will be generally used for the HVAC air return and the total space is also used to obfuscate piping, cabling, and ducts, that's why the middle-to-middle spacing of curved lighting systems would have to be in incremental increases of 4 ft.

where’s the list ? the question is incomplete

e/c, where e is its total energy