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.

Article: You will spend hours at the gym training your body, but do you ever think about training your brain? Here are four ways to keep your brain mentally active and healthy. Play games Your brain is a muscle just like the ones in the rest of your body. If you don't use it, you will lose it. For example, doing a crossword puzzle once a day is a good way to give the brain the exercise it needs. This kind of puzzle is good at training different areas of the brain. But remember that when doing this puzzle, you should gradually increase the complexity ; otherwise, it will become repetitive and stop being mentally challenging. Talk aloud Although it might sound strange to you, talking aloud is a great way to _ your memory. A good exercise to try is counting to 99 in threes as fast as you can. Be a team leader If you find you can do your work very easily, you should find a new task. It is important that your job should be mentally challenging. Complex jobs are often more challenging and make the brain work a little harder. Learn a language A study found that, compared with those who could speak only one language, the people who could speak two languages had a slower rate of cognitive decrease. Learning a new language is very challenging and it makes sure that different areas of the brain have their lights on.
Question: What is the main idea of this passage?
Options: (A) Why you should train your brain. (B) How to be a good team leader. (C) Training the brain is more important than training the body. (D) How to make your brain healthy and work actively.
D