78. The coefficient of linear expansion of brass and steel are

The coefficient of linear expansion of brass and steel are α₁ and α₂ . If we take a brass rod of length l₁ and steel rod of length l₂ at 0°C, their difference in length (l2-l1) will remain the same at a temperature if

(a)   a_1l2-a_2l1                                     (b)  a_1l2 ²-a_2l1 ²

(c)  a₁  ² _l1 -a₂  ² _l2                            (d) a_1l1-a_2l2

Answer-d