Last summer we visited Yellowstone National Park. It is very lively and ____.We wanted to see the beautiful views there. It is the oldest national park in the world.____ visitors must be careful not to leave food in their tents at night，because there are ____of bears in the park in the season. If the bears are ____, they might come into the tent to eat it. So people must take care of ____ at night. It is important to us.