Task: Question Answering
Input (English): Question: The work of expanding a gas (W): 
 Options:  <1> It is a state function. <2> Equivalent to the heat associated with the process (Q). <3> Against constant pressure it is calculated with W = -Pext ΔV. <4> In a reversible isothermal process it is calculated with W = -nRTΔV. <5> Equivalent to the internal energy change during the process (ΔU)
Output (English): 3