The process of impregnation in powder metallurgy technique is best described by which of the following?
(a) After sintering operation of powder metallurgy, rapid cooling is performed to avoid thermal stresses
(b) Low melting point metal is filled in the pores of a sintered powder metallurgy product
(c) Liquid oil or grease is filled in the pores of a sintered powder metallurgy product
(d) During sintering operation of powder metallurgy, rapid heating is performed to avoid sudden produce of high internal pressure due to volatilization of lubricant