Between 1980 and 2000 the sea otter population of the Aleutian Islands declined precipitously. There were no signs of disease or malnutrition, so there was probably an increase in the number of otters being eaten by predators. Orcas will eat otters when seals, their normal prey, are unavailable, and the Aleutian Islands seal population declined dramatically in the 1980s. Ttherefore, orcas were most likely the immediate cause of the otter population decline. Which of the following, if true, most strengthens the argument? ['The population of orcas in the Aleutian Islands has declined since the 1980s.', 'Seals do not eat sea otters, nor do they compete with sea otters for food.', 'Most of the surviving sea otters live in a bay that is inaccessible to orcas.', 'The population of sea urchins, the main food of sea otters, has increased since the sea otter population declined.']