An Oxygen Cylinder Used by Scuba Divers Contains 21% of Oxygen

An oxygen cylinder used by scuba divers contains 21% of oxygen (mimicking air) . Pure oxygen at a pressure above 1 atmosphere is toxic. If the undersea pressure increases by one atmosphere every 33 feet, how deep can a diver using the oxygen cylinder dive before the oxygen gets toxic?

A) 99 ft.
B) 112 ft
C) 174.67 ft
D) 157ft
E) 225 ft