The mole fraction of argon in dry air near sea level is 1.818 × 10-5 where the molar mass of argon is 39.948. The concentration of argon in the atmosphere is ________%.

1.818 × 10-5
1.818 × 10-3
7.23 × 10-4
9.09 × 105
9.09 × 10-4