One mole of helium is adiabatically expanded from its initial state $({\mathrm{P}}_{\mathrm{i}},{\mathrm{V}}_{\mathrm{i}},{\mathrm{T}}_{\mathrm{i}})$ to its final state $({\mathrm{P}}_{\mathrm{f}},{\mathrm{V}}_{\mathrm{f}},{\mathrm{T}}_{\mathrm{f}})$ . The decrease in the internal energy associated with this expansion is equal to

• A

  ${\mathrm{C}}_{\mathrm{V}}({\mathrm{T}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{f}})$

• B

  ${\mathrm{C}}_{\mathrm{P}}({\mathrm{T}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{f}})$

• C

  $\frac{1}{2}({\mathrm{C}}_{\mathrm{P}}+{\mathrm{C}}_{\mathrm{V}})({\mathrm{T}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{f}})$

• D

  $({\mathrm{C}}_{\mathrm{P}}-{\mathrm{C}}_{\mathrm{V}})({\mathrm{T}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{f}})$