Write a multi-choice question for the following article, with the given choices and answer: Article: New research shows how kids' brains reorganize as they learn math. All the time you spent memorizing multiplication tables may have made you a better mathematician, according to a new study. A team of scientists from Stanford University, in California, have shown how the brain reorganizes itself as kids learn math. After a certain amount of time spent practicing math, kids can put away the calculator . They don't even need to count on their fingers. They simply know the answers to subtraction , addition, and multiplication facts. The quicker kids can recall basic math facts, the easier it is for them to solve more complicated math problems. The Stanford University researchers observed the brain activity of 28 students aged 7to 9 for the study. They took scans of the students' brains as the students solved math calculations without the help of a calculator, pen or paper. A calculation--three plus four equals seven, for example--flashed on a screen. The students pushed a button to say if the answer was right or wrong. The scientists also recorded the response speed, and what parts of the brain became active as the kids pushed the button. These observations showed a process called fact retrieval . Rather than using their fingers to count, or writing out answers on a piece of paper, the students pulled the answers from memory. It's as if the answers to basic math problems are kept in a long-term storage area in the brain, which was built from repetition. "Experience really does matter," said Dr. Kathy Mann Koepeke. Children make the shift from counting to fact retrieval when they are 8 to 9 years old, the study shows. This is the time when most students are learning basic addition and subtraction. When kids have basic math facts memorized, the brain has more free space to learn more complicated math. This process has benefits for the future. The study shows as kids grow older, their answers rely more on memory and become quicker and more accurate. Less brain activity is devoted to counting. Some children make... Options: A They recorded the students' brain activities. B They pushed a button linked to the students. C They noticed whether they used a calculator. D They found out who responded most quickly. Answer: A They recorded the students' brain activities. Question:
The answer to this question is:
What did the researchers do when students worked out the given problems?