Mechanical draft venting systems of other than direct-vent type shall terminate at least _____ below, __________ horizontally from, or _________ above any door, operable window, or gravity air inlet into any building.
a) 6 inches, 12 inches, 18 inches
b) 12 inches, 24 inches, 36 inches
c) 18 inches, 36 inches, 48 inches
d) 24 inches, 48 inches, 60 inches