Ventilation supply systems shall deliver the required rate to the breathing zone, which extends to feet from the enclosing walls.

The correct answer is "2".

Explanation:

The missing options of this question are:

a) 2

b) 3

c) 4

d) 6

The correct answer is option a) "2".

According to the Federal Emergency Management Agency (FEMA) of the United States of America, a breathing zone must be established in order to ensure a proper ventilation supply. The breathing zone is a region that must be kept among systems and the enclosing walls or the floor. In the case of enclosing walls, at least two feet (600 millimeters) must be kept. Regarding the floor, at least three feet must be kept above the floor.

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