18. Assume that CH4 behaves as an ideal gas and its heat capacity is mainly contributed by translational and rotational motion but none of vibrational ones. Consider the irreversible compression of 1 mole of CH4 from an initial state at P₁ = 1 bar and T₁ = 300 K to a final state at P2 = 4 bar, T2 = 400 K, against a constant external pressure of Pext = 4 bar. What is the heat transfer q during the process? (A) 2494.2 J (B) 4157.0 J (C) 6651.2 J (D) -6651.2 J (E) -4157.0 J