On the top of Mount Everest, the total atmospheric pressure is reduced to 33.73 kPa (about one-third of its value at sea level).

Assume that the fraction of the atmosphere that is made up of oxygen is the same at all elevations.

If the partial pressure of oxygen at sea level is 21 kPa, what is the partial pressure of oxygen on the top of Mount Everest?

A 12.73 kpa

B 11.2 kpa

C 54.73 kpa

D 7.0 kpa