In the following passage, some words have been deleted. Fill in the blanks with the help of the alternatives given. Select the most appropriate option for each blank.

Generally, dolphins sleep (1) ______ only one brain hemisphere in slow-wave sleep at a time, thus (2) ______ enough consciousness to breathe and to watch for possible (3) ______ and other threats. Early (4) ______ stages can occur simultaneously in both hemispheres. In captivity, dolphins seemingly enter a fully asleep state where both eyes are closed and there is no response to mild external (5) ______.