In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.

Let me give you an example: Article: Homework can put you in a badmood , and that might actually be a good thing. Researchers from the University of Plymouth in England doubted whether mood might affect the way kids learn. To find out the answer, they did two experiments with children. The first experiment tested 30 kids. Someshapes were hidden inside a different, larger picture. The kids had to find the small shapes while sitting in a room with either cheerful or sad music playing in the background. To test their mood, the scientists asked the kids to point to one of five faces, from happy to sad. Children who listened to cheerful music tended to point to the smiley faces while the others pointed to the unhappy ones. The researchers found that sad kids took at least a second less to find the small shapes. They also found an average of three or four more shapes. In the second experiment, 61 children watched one of two scenes from a film. One scene was happy, and the other was sad. Just like in the first experiment, kids who saw the sad scene acted better compared to the others. The researchers guessed that feeling down makes people more likely to focus on a problem or difficult situation. Not all scientists agree with them, however. Other studies argued that maybe, that cheerful music in the first experiment distracted kids from finding shapes. While scientists work on finding out the answers, it still might be wise to choose when to do your tasks according to your mood. After eating a delicious ice cream, for example, write an essay.
Answer: whether mood affects their learning ability
The answer to this example can be: Researchers did experiments on kids in order to find out _ .
Here is why: It is clearly mentioned in the article that to the check effect of mood on children is the main purpose of the experiment.

OK. solve this:
Article: Global warming has a great influence on species all over the world. Polar bears are particularly at risk because their habitat is increasingly threatened by global warming. Lots of scientific research has shown that polar bear numbers are on the decline, and that if action is not taken quickly, polar bears may die out from the Earth. Pack ice is decreasing globally. In the wild, polar bears spend much of their lives wandering pack ice in search of food. In 2009, several polar bear deaths by drowning were recorded, which had never been heard of before. The polar bears had been trapped on isolated areas of floating ice, and had tried to swim for land. Due to the shrinkage of the ice, the polar bears tired and drowned before reaching the shore, though they are rapid and aggressive swimmers. This problem will only grow as the pack ice shrinks. The shrinkage of the ice has other consequences for polar bears. They are losing their natural prey , who are also affected by the shrinkage in habitat. Land animals also do not provide the high-fat diet that polar bears need, which will lead to starvation. Starving mothers will not be able to provide their babies with the nutrition they need, which will further contribute to the decline in polar bear numbers. While searching for food and habitat, polar bears have also fought with humans. With the disappearance of the sea ice, polar bears have begun to show up around inhabited areas looking for food and shelter. But they often end up being killed by hunters because they present a threat to human communities. The combination a deadly factors brought about by global warming put polar bears at a very high rate of risk. A global effort is needed to act against global warming before it is too late for polar bears, as well as many other species that call Earth home.
Answer: they were seriously hurt when hunting
Answer:
The reason why several polar bears died in 2009 is that _ .