Read and Complete

Choose the word that best completes each blank.

An octopus has an unusual body. It has three hearts: two pump blood through the gills, while the third sends blood to the rest of the body. Its blood is blue rather than red because it carries oxygen using a copper-based substance instead of the iron-based one found in human blood. The main heart stops beating when the octopus swims, which is one reason many octopuses prefer to crawl along the seafloor.

An octopus has —threetwofive hearts, and its blood looks —blueredgreen because it carries oxygen with copper rather than iron.