Hi- we were given this question and I have no clue how to figure it out...please help!

Which gas molecules have the greatest average speed at STP?
[a] SO2( sulfur dioxide)
[b] NH3( ammonia)
[c] O2( oxygen)
[d] N20 (Dinitrogen oxide)

Ok, at stp, that means they all have the same average kinetic energy. But KE= 1/2 mv^2, so the one with the greatest speed must have the least mass.

thank you so much