The type of study that this is is observational study.
He is observing his team, each one of the members, in order to see how long they can hold their breath - which is why this is an observational study. Nothing is random here, there is no hypothesis, and there is no double blind, which means that the other options are incorrect.
Arab–Byzantine wars were a series of wars between the mostly Arab Muslims and the East Roman or Byzantine Empire between the 7th and 11th centuries AD, started during the initial Muslim conquests under the expansionist Rashidun and Umayyad caliphs in the 7th century and continued by their successors until the mid-11th century.
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Explanation:
