All oceangoing ships carry seawater ballast tanks whose weight improves stability. To maintain the ship' s proper stability, water must be pumped out of these tanks when cargo is loaded and into them when cargo is unloaded. As a result, sea creatures often get into the tanks and are then inadvertently deposited into new habitats, where they can wreak ecological havoc. One viable way of addressing this problem would be to empty and then immediately refill the tanks in midocean, since midocean creatures and coastal sea creatures usually cannot survive in one another' s habitats. Which one of the following is an assumption the argument requires? ['Sea creatures have rarely, if ever, wreaked ecological havoc in a new habitat, unless they have been able to survive in that habitat after having been deposited there by oceangoing ships.', 'There are at least some oceangoing ships whose stability could be adequately maintained while emptying and refilling their ballast tanks in midocean.', 'Currently, seawater is pumped into or out of the ballast tanks of oceangoing ships to maintain proper stability only when unloading or loading cargo.', "An oceangoing ship's ballast tanks could be emptied and refilled in midocean only in conditions of calm air and flat seas."]