Thermal conductivity of a material may be defined as the________________?

A. heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces

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 1C

C. quantity of heat flowing in one second through one cm cube of material when opposite faces ^re maintained at a temperature difference of 1C

E. none of the above