Consider the Following Reaction (Assume an Ideal Gas Mixture)

Consider the following reaction (assume an ideal gas mixture) . 2NOBr(g) 2NO(g) + Br₂(g)
A 3) 0-L vessel was initially filled with pure NOBr, at a pressure of 4.0 atm, at 300 K.
After equilibrium was established, the partial pressure of NOBr was 2.1 atm. What is K_p for the reaction?

A) 0.78
B) 0.26
C) 1.56
D) 1.72
E) 1.1