The characteristics of two methods of heat transfer are listed below: method a: no material medium is required and heat is transferred in all directions method b: material medium like gas or liquid is required and heat is transferred only in the vertically upward direction which statement is correct? method a is radiation and method b is conduction. method a is radiation and method b is convection. method a is conduction and method b is radiation. method a is convection and method b is radiation.