According to 14 CFR Part 91.211, under what conditions must occupants of a pressurized cabin aircraft be provided with supplemental oxygen?
a) When flying at cabin pressure altitudes above 12,500 feet (MSL)
b) When flying at cabin pressure altitudes above 15,000 feet (MSL)
c) When flying at flight altitudes above flight level 250
d) When flying at flight altitudes above flight level 350