Consider the following gas phase reaction: 2NO(g) + O2(g) --> 2NO2(g)

400. mL of NO at STP is reacted with 500. mL of O2 at STP. Calculate the volume of the reaction mixture at STP after the reaction goes to completion.

A.700 mL
B.800 mL
C.900 mL
D.1300 mL
E.100 mL

The answer is A. 700 mL, can someone please explain how they arrived at that answer.