How do you figure out this problem?

Two 500 mL flasks contain gas at STP. Flask A contains carbon monoxide, and flask B contains carbon dioxide.


Choose the true statement.


Flask A has less mass, but the same number of particles as B.


Flask A has less mass, and fewer particles than B.


Flask A has more mass, but fewer particles than B.


Flask A has more mass, and more particles than B.





I really need help thanks:)How do you figure out this problem?
Flask A has less mass, and fewer particles than B.