(a) Express the cost (in dollars)of making x small boxes, y medium boxes, and z large boxes as a function of three variables: C=f(x,y,z).f(x,y,z)=(b) Find f(4,000,3,000,6,000) and interpret it.This is the cost for makingsmall boxes, medium boxes, andlarge boxes.(c) What is the domain off?Each ofx,y and z must be a positive integer or zero.Each ofx,y and z can be any non-negative real number.Each ofx,y and z can be any real or complex number.Each ofx,y and z can be any real number.Each ofx,y and z must be a negative integer or zero.