28. In an adiabatic expansion of a gas initial and final temperatures

are T₁ and T₂ respectively, then the change in internal energy of the gas is

(a)    (R/ϒ-1) (T₂-T1)                 (b) (R/ϒ-1) (T1-T₂)

(c)     R(T1-T₂)                            (d)  Zero

Ans. a

(14)