An Oxygen Tank Kept at 20 $\degree$ C Contains 28

An oxygen tank kept at 20 $\degree$ C contains 28.0 moles of oxygen and the gauge reads 31.0 atm.After two weeks,the gauge reads 10.5 atm.How many moles of oxygen were used during the two-week period?

A) 9.25 moles
B) 7.5 moles
C) 20.5 moles
D) 18.5 moles
E) 9.48 moles