Answer:a. gastropods, bivalves
Explanation:mollusca is a phylum which consist of organisms whose body are bilaterally symmentrical (can be dived into two equal mirror part through a unique plane)and segmented, except the gastropods which are asymmetrical.
Mollusca possess a ventral body wall and is modified into a muscular foot for lomotion.they use radula for feeding , except the bivalves which are filter feeders
They possess gills/ctinedia for respiration.they may also use lungs or body surface for respiration.
Snail is an example of gastropoda.they are the most populous and diverse class of mollusca.other examples include slugs, periwinkle etc.
Oysters is an example of bivalva.they have two calcerous valves held together by a ligament.they have no head and their body is compressed in their shell.examples oysters and clams.
