Consider the reaction between nitric oxide and oxygen to form nitrogen ... ...

Consider the reaction between nitric oxide and oxygen to form nitrogen dioxide. If 4 liters of nitrogen dioxide were formed, how many liters of nitric oxide and oxygen must have reacted?  
(A) 2 L of NO & 1 L of O2
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in 06. Gases