Why is a gastropod's shell not consid-
ered an exoskeleton?

Gastropod's shell not considered an exoskeleton because they do not shed their exoskeleton.

Explanation:

Gastropods like snails have unique shell structure which protects them with the hard covering and by providing a camouflage appearance.

The shells of the gastropods contain protein and calcium which facilitates growth of the shells. The gastropod shells often grow from bottom to top layering into three layers – the inner nacre, prism, and the periosteal layer.

The exoskeleton of animals like the arthropods exclusively contains the polysaccharide chitin. The exoskeleton will periodically shed because it is acellular and does not grow in line with the growth of the organism.  

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.

c is the answer to this question. altitude affects the weather because air molecules evaporate.