A sealed 10.0\,L flask at 298\,K contains a mixture of three ideal gases. The total pressure is 1.20\,atm. The partial pressures of N2 and O2 are 0.50\,atm and 0.30\,atm, respectively. The remainder of the pressure is due to argon. What is the mole fraction of argon in the mixture?
a) 0.67
b) 0.40
c) 0.33
d) 0.58