[阅读理解]

【题干】Directions :There are five reading passages in this part. Each passage is followed by four questions. For each question there are four suggested answers marked A, B, C and D. Choose the best answer and blacken the corresponding letter on the Answer Sheet.
Scientists have shown that exercise is linked to brain changes throughout all stages of life and can help the brain develop and stay healthy. Babies ，for example，need regular exercise to form connections in the brain. In children， research suggests that exercise improves attention, focus and school performance. In the elderly ，exercise has been shown to help slow memory loss.
Then how does exercise help the brain? Over the last 20 years，scientists have learned that exercise can help keep the mind sharp in a number of ways. Exercise improves blood flow to the brain. The blood carries oxygen, contributing to more efficient connections between brain cells. Increasing blood flow is one way that exercise can improve mental abilities. The positive effects of exercise on the brain can be seen in babies, pre - teenagers and adults.
Babies are in near-constant movement， which is extremely important for development. This movement not only strengthens their muscles, but also helps their brains form connections. The process continues throughout life but is most intense in infancy ( 婴儿期) and toddlerhood ( 学步期)，when children are mastering brand-new skills like sitting, standing, walking, running and jumping.
Exercise is also healthy for pre-teenagers ’brains. In fact, some research suggests that regular exercise can improve school performance. A study found that the effects depended on how much kids exercised. The more days the children attended the exercise program, the more their focus improved.
Finally, exercise helps keep the mind sharp during adulthood. Research suggests exercise can increase the size of the hippocampus, an important area of the brain, which becomes smaller with age ，and can increase levels of a protein ( 蛋白质) that aids the growth of new brain cells. This can help prevent older adults from losing mental abilities and memory.

【题目】How does exercise sharpen the mind according to Para. 2?
A. By increasing the brain size.
B. By decreasing the protein level.
C. By extending the time of concentration.
D. By strengthening brain cells' connections.

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