Polar bears are born in dens on land and their mother stays with them for the whole winter without eating anything. The mother and the babies come out from the dens around March,  _ from a long winter and must immediately go hunting to find food to stay alive. Now, the only hunting they know is to catch seals from floating ice in the open ocean. In March, polar bear mothers and babies have to swim from the land in Spitsbergen to the edge of the polar ice cap. Normally this is a short swim, since in March the polar ice cap is very close to the land, and the polar bears, and even the babies, are good swimmers. However, last year we saw that the polar ice cap had become smaller in size due to global warming.  Swimming hundreds of miles, especially when they are hungry and have babies following them, is really hard, and many bears are dying because of starvation. The polar bear population in the Spitsbergen area was once threatened by hunters. In the 1970s, a ban on hunting was imposed  and the polar bear population has been steadily increasing from just a few hundred to about 2,000 to 3,000. This was a great success story. However, now, because of global warming, the population is again in danger. If we do not stop making so much carbon dioxide, we will lose these beautiful animals forever.    The polar bears are just one example of animals that have become endangered because of man's appetite for energy obtained by burning fossil fuels. All life on earth could be in danger unless we change our ways. What can we do? To reduce the amount of carbon dioxide in the air we must save and make energy without burning coal and oil. Instead, we can use the sun or the wind to generate electricity. We must also cut down on needless driving and turn off things that use electricity when we are not using them. In addition, we have to stop cutting down forests and start planting more trees. Many bears die in the ocean because   _  .
they are not very good swimmers they don't have enough strength to swim to the polar ice cap the number of seals has decreased so they can't get enough food. they are hunted by human beings