Q. Thermal conductivity of a material may be defined as the

• (A) quantity of heat flowing in one second through one cm cube of material when opposite faces ^re maintained at a temperature difference of 1°C
• (B) quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C
• (C) heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
• (D) all of the above