Filter aids like asbestos, kieselguhr, diatomaceous earth etc. are used to increase the porosity of the final filter cake & reducing the cake resistance during filtration. Filter aid is

• A. added to the feed slurry.
• B. precoated on the filter medium prior to filtration.
• C. separated from the cake by dissolving solids or by burning it off.
• D. all 'a', 'b'&'c'.