Much of Tibet Lies at Altitudes Over 18,000 Feet Where

Much of Tibet lies at altitudes over 18,000 feet where the pressure is about 500 mb.At such altitudes,the Tibetans are above roughly

A) 10 percent of the air molecules in the atmosphere.
B) 25 percent of the air molecules in the atmosphere.
C) 50 percent of the air molecules in the atmosphere.
D) 75 percent of the air molecules in the atmosphere.
E) 90 percent of the air molecules in the atmosphere.