For the Isomerization Reaction: Butane ⇌ Isobutane

For the isomerization reaction: butane ⇌ isobutane
K_p equals 25 at 500°C.If the initial pressures of butane and isobutane are 20.atm and 0.0 atm,respectively,what are the pressures of the two gases at equilibrium?

A) P(butane) = 0.77 atm and P(isobutane) = 19.2 atm
B) P(butane) = 0.80 atm and P(isobutane) = 20. atm
C) P(butane) = 19.2 atm and P(isobutane) = 0.77 atm
D) P(butane) = 20 atm and P(isobutane) = 0.80 atm