已知某生产厂家的年利润y（单位：万元）与年产量x（单位：万件）的函数关系式为$y=-\frac{1}{3}{x}^3+81x-234$，则使该生产厂家获得最大年利润的年产量为（   ）.

银镜反应是检验醛基的重要反应，教材对该实验的操作条件只是粗略的描述．某同学进行如下研究：

如图表示一组密封的玻璃容器，在适宜的光照和温度条件下，绿色植物生长在潮湿土壤中，请据图回答问题：

    ①Mary is reading something about how to keep healthy on the Internet.

    Good sleep is important for your health. You need about eight hours' sleep a night. Don't eat or read in bed. A cup of milk before sleeping may help you get a good sleep. Go to bed at the same time before midnight(午夜) and get up at the same time every morning. For more information, please visit www.sleep.com.

    How often do you exercise? If you want to stay healthy, try to ②每天锻炼30分钟，每周三四次。For more information, please call Health Line at 180-2000.

    Studies show that the cold or flu virus(流感病毒) can live on our hands for long. So you should wash your hands often with soap and water. If you want to know more, please call Health Line at 180-1313.

    ③Brush your teeth twice a day and see a dentist at least once a year. The oral examination(口腔检查) is not only for the health of teeth, but also for the whole body. Please visit www.mydr.com for more information.