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Polar bears are one of the largest land mammals（哺乳动物）on Earth. You'll find them in northern Greenland, Norway and Canada. Polar bears spend most of their lives in, on or around water. They are excellent swimmers, but they prefer to stay on top of the ice. Their favorite food is seals（海豹）.
Seals can be hard to catch. So polar bears must hunt with great patience. Luckily, their white coloring helps them blend in with the icy surroundings.
However, polar bears' hairs aren't actually white. Their long outer hairs are mostly hollow（空心的）and transparent. The air spaces in the hairs scatter（分散）light of all colors. For that reason,
we look at polar bears and see the color white.
Some scientists believe the polar bear was once a close relative to the brown bear. They think that, over time, polar bears moved to the Arctic（北极地区）. There, they adapted to their surroundings. Slowly, they developed fur that would help them "hide" in the Arctic ice.
Polar bears are good at 1. , but they prefer to stay on top of the ice. They mainly eat seals, which can be hard to catch. Fortunately, polar bears' white coloring helps them 2. in the icy surroundings. However, polar bears' hairs aren't actually white. Their long outer hairs are mostly hollow and transparent. The air spaces in the hairs scatter 3. of all colors. Therefore, we look at polar bears and see the color white.
Some scientists believe the polar bear was once a close 4. to the brown bear. They think that, over time, polar bears moved to the Arctic. There, they 5. to their surroundings slowly.

1. swimming 2. hide 3. light 4. relative 5. adapted