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.
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
Example solution: Researchers did experiments on kids in order to find out _ .
Example explanation: It is clearly mentioned in the article that to the check effect of mood on children is the main purpose of the experiment.

Problem: Article: Americans like to visit the national parks. In 1996, over 250,000,000 people visited areas which are managed by the National Park Service. Experts say that by 2012, 500,000,000 people per year will visit the parks. As a result, people are trying to think of ways to protect the parks from the crowds. The crowding of the parks has caused several problems. One is that there is not enough space for all of the people who want to use the parks. To deal with this problem, you may have to book your tickets a few months ahead of time. The Park Service may also raise entrance fees   . Another problem caused by increased park use is pollution. Some garbage and waste can be cleaned up, and the cleanup can be paid for by increased entrance fees. To cut down on noise pollution and air pollution, the number of cars allowed in parks may be limited. If this happens, people may use buses to travel around the parks. Officials may also limit the use of jet-skis, snow-mobiles, motor-boats, and sightseeing helicopters and planes. The parks may also be in danger because of development that is going on around them. For example, around Yellow Stone Park, many hotels, golf courses, resorts, and other tourist attractions have been built. Some of these developments have effect on areas that are used by animals. People will have to learn to agree about ways to protect the parks. If they do not, there will soon be no reason to visit these national treasures. 
 Answer: may be doubled

Solution: According to experts, by 2012 the number of visitors   _  .