Read the following article and answer the question. Article: Have you ever trained your brain ? Now there are some new suggestions, such as eating dark chocolate and having cold meat for breakfast. You should also avoid watching soap operas, spending time with people who complain or follow fat-free diets, according to a new book on getting "brain-fit". Many of the suggestions in Teach Yourself Training Your Brain are surprising. The book says to read out loud while holding a baby. Writers Terry Horne and Simon Wootton say their advice is from the latest research by experts around the world. "For years we have thought our brain capability is decided by our genes . But it's now clear that it depends on our lifestyle. What we eat and drink, how we learn at school and what type of feelings we have are all important," said Horne. The book talks about the most up-to-date thinking in science about how diet, the environment, stress and other sides of modern life affect our brain capacity. It also offers advice and exercises. _ Horne believes that people need to make changes to their lifestyles to increase their brain capacity Happiness, confidence and an optimistic nature help the brain, he added. "So mix with people who make you laugh, or share the same interests as you. And avoid people who complain," said Horne. "People who are negative will make you sad, which means you won't achieve things." Question: What's the book Teach Yourself Training Your Brain mainly about? Answer:
Select from: a. How people's brain capability works b. Exercises to improve brain capability c. Some scientific and healthy diets d. How to improve people's brain capability
d.