Teacher: In this task, you're given an article, a question which often contains a blank and four options (associated with "A", "B", "C", "D"). Your task is to find the correct answer (from the given options) for the question from the given article and return one of the options from "A", "B", "C", and "D". Do not generate anything else apart from one of the following characters: "A", "B", "C", "D". There is only one correct answer for each question.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Article: Homework can put you in a bad mood , 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.
Question: Researchers did experiments on kids in order to find out _ .
Options: (A) how they really feel when they are learning (B) whether mood affects their learning ability (C) what methods are easy for kids to learn (D) the relationship between sadness and happiness
Solution: B
Reason: It is clearly mentioned in the article that to the check effect of mood on children is the main purpose of the experiment. So, B is the correct answer.

Now, solve this instance: Article: Long ago, little attention was paid to a person's birthday. Even though everyone knew that people grew older, they had no way of correctly marking time. It wasn't until the early people learned more about how time passed that they made a note of time changes. Then they developed a calendar and began to celebrate special events such as birthdays. When birthdays first started to be celebrated during the early days, common people, especially children, hardly celebrated their birthdays. Today, birthdays are celebrated by the young and the old alike all around the world. Many countries have different customs from ours of celebrating birthdays. At the same time there are a lot of people that celebrate birthdays in quite similar ways as we do. Some of the countries are very similar to the United States, such as Austria, Denmark, Ireland, Sweden and Switzerland. Customs within countries can be affected by things like language, religion, geographic location, and economic conditions. No matter what the customs are, however, they are always followed by the majority of the people in each country. For example, not all children in the US have birthday parties, candles or birthday cakes, but most of them do. However, the reasons why we celebrate birthdays and use different symbols are very much the same to everyone. The reason why we have birthday parties goes back to long time ago. At that time people believed that good and bad spirits appeared when a child was born and influenced that child through his / her life. The belief brought about the custom of having birthday parties. They believed that surrounding the birthday person with friends, family and good wishes would frighten the bad spirits away.
Question: According to the passage, when did people begin to celebrate birthdays?
Options: (A) When they learned that no one could stop time. (B) Before they invented a calendar. (C) Before they learned how time went by. (D) After they had a right way of marking time.
Student:
D