(a) any person who engages in the handling of skim milk or butterfat, which was received at a pool plant, a partial pool plant, a pool unit or a partial pool unit or at a plant approved by any health authority as a source of skim milk or butterfat for disposition as fluid milk products in the marketing area;

(b) any person who engages in the handling of fluid milk products, all or a portion of which is shipped to, or received in, the marketing area; or

(c) any cooperative association with respect to milk which it causes to be delivered from producers to any other handler for the account of such association and for which such association receives payment.