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Polar bears off Alaska normally hunt and raise their young on ice sheets that float on the ocean.But as the ice has melted,the polar bears have been forced to spend more time on land.There,they have begun to frequent① beaches,feeding on the remains② of whales caught by native hunters.For polar bears,this food is less nutritious③ than seals that they normally catch on ice sheets.The shrinking ice has also forced more polar bears into the ocean.In the past,they only had to swim short distances between ice sheets.But as the ice has shrunk,polar bears have been forced to swim longer and longer distances in the open ocean.This poses④ a severe danger during rough weather,and an increasing number of drowned polar bears have been observed.