The internal energy of a gas is given by ( u = 3x ). It expands from ( V₀ ) to ( 2V₀ ) against a constant pressure ( P₀ ). The heat absorbed by the gas in the process is:
A) ( 3P₀V₀ )
B) ( 6P₀V₀ )
C) ( 9P₀V₀ )
D) ( 12P₀V₀ )