题目

For the first time in 10 years，the United States government has changed its guidance on how much exercise people need to stay healthy and when they should start.For children and teensThe new guidance states that the most important time for children to begin exercising is between the ages of 3 and 5. 【1】 That could be light activities，moderate activities，or even vigorous(剧烈的)activities.Doctors say it is important to start young.They add that from birth to age 5，a child’s brain develops more than at any other time in life. 【2】 Also，children who start exercising at a you18．根据玉米种子结构示意图，回答下列问题：（1）胚是新植物的幼体，它是由图中①②③④（填图中的标号）组成的．（2）玉米种子具有1片子叶，所以玉米属于单子叶植物．（3）玉米种子中将来发育成茎和叶的结构是[③]胚芽，遇到碘液变蓝的结构是[⑥]胚乳．