Circulatory systems in molluscs: a) are closed in all species of molluscs. b) are open in species of large-sized molluscs and are closed in species of small-sized molluscs. c) are open in species of small-sized molluscs and are closed in species of large-sized molluscs. d) are open in all species of molluscs. e) are open or closed without regard to body size.

b) are open in species of large-sized molluscs and are closed in species of small-sized molluscs.

Explanation:

Class Cephalopoda are the largest molluscs and are known for open circulatory system(enclosed blood vessels do not totally contain blood as blood is pumped with the aid of blood vessels from the gills into body cavities also known as hemocoels by the heart)

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.