Amantle and mantle cavity are present in
class oligochaeta
class bivalvia
class polychaeta
class hirudinea

The correct answer is class bivalvia.

Explanation:

The phylum Mollusca comprises seven classes, that is, Aplacophora, Bivalvia, Monoplacophora, Gastropoda, Cephalopoda, Scaphopoda, and Polyplacophora. The mollusk is categorized by the existence of a flap-like tissue over the visceral mass called the dorsal mantle that produces a space called the mantle cavity. The visceral mass comprises all the internal organs of these species. The mantle cavity, that is, produced within the mantle forms in an autonomous way from the coelomic cavity.

a polymer is a chainlike molecule made up of small molecular units. these small molecular units are called monomers. monomers of polymer can be joined together either naturally or synthetically.  

is this for the e2020 class