Many people may not believe it, but this is what is happening: polar bears are dying out! They are dying because they are losing a great deal of habitat due to human incursion（侵入）and global warming.  They are dying because of starvation as well.  With the sea ice they depend on for survival shrinking, it is becoming increasingly difficult for them to find food.
An ecosystem is highly dependent on all of its parts, and removing even a single species can result in serious consequences.  Polar bears are at the top of the predators（捕食性动物）where they live.  They play an important part in the ecosystem.  They feed on seals, fish and deer.  Several other animals depend on the kill of polar bears.  The failure to save polar bears might result in loss of these predators.
In addition, all large predators perform a very important function in their habitat: helping to control the populations of other species.  When an animal group loses its natural predators, its population will rise rapidly, having a direct result in population decrease of other groups.  A huge boom in population of Arctic seals would have a direct effect on the prey(捕食) of seals, and could reduce numbers of certain types of fish.
This could also have a direct effect on humans, who might have to compete with Arctic seals for food supplies from fish. Lack of polar bears could eventually affect human food supply. Human beings shall save polar bears to ensure the food supply.
There is no doubt that we should save polar bears.  However, there is still another question: can they be saved? This is a matter that scientists debate because of rapid degradation（恶化）of the polar bear’ environment in recent years due to warming temperatures.  It is estimated that if the bears cannot adapt to changing climates, they might die out before the end of the 21^st century, with about 50% of the animals disappearing by 2050. So time is running out!
There are some things that may help slow down global warming and help delay extinction of polar bears.  These include finding new energy sources that don’t give off greenhouse gases and not making any more incursions into polar bear’s territory.
Which is NOT the threat to the survival of polar bears according to the passage?

A．Human’s killing.	B．Global warming.
C．Habitat loss.	D．Lack of food.

Why do we need to save polar bears?

A．For the predators in the ecosystem.

B．For the seals in Arctic Circle.

C．For our own source of food.

D．For our environment.

If bears can’t adapt to changing climates, we will probably have lost     _____   by the middle of the 21^st century.

A．half of polar bears

B．most of polar bears

C．90% of polar bears

D．all of polar bears

We can infer that    _____   .

A．nothing can prevent the extinction of polar bears

B．drilling for oil in the Arctic should be encouraged

C．polar bears can surely adapt to changing climates

D．using solar energy is good for saving polar bears