How do you figure out this problem?

I think " Flask A has less m but the same # of particles as B" because carbon monoxide(CO) has molecular weight of 28 gr/mol, but carbon dioxidle (CO2) has 44 gr/mol, therefore each mole of CO2 weighs more than CO, but because one mole of all gases at STP occupy the same volume of 22.4 Liter/mol, therefore Flask A & B have the same # of particles.