如图，小瓷碗漂浮在水面上，倾斜后能沉入水底，关于这一情景分析正确的是（  ）

解答题唐代大诗人李白喜好饮酒作诗，民间有“李白斗酒诗百篇”之说．《算法统宗》中记载了一个“李白沽酒”的故事．诗云：

注：古代一斗是10升．

大意是：李白在郊外春游时，做出这样一条约定：遇 见朋友，先到酒店里将壶里的酒增加一倍，再喝掉其中的19升酒．按照这样的约定，在第3个店里遇到朋友正好喝光了壶中的酒．

（1）列方程求壶中原有多少升酒；

（2）设壶中原有a₀升酒，在第n个店饮酒后壶中余a_n升酒，如第一次饮后所余酒为a₁=2a₀﹣19（升），第二次饮后所余酒为a₂=2a₁﹣19=2（2a₀﹣19）﹣19=2²a₀﹣（2¹+1）×19（升），…．

①用a_n﹣1的表达式表示a_n，再用a₀和n的表达式表示a_n；

②按照这个约定，如果在第4个店喝光了壶中酒，请借助①中的结论求壶中原有多少升酒．

根据短文内容，从短文后的七个选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

    Sometimes willpower is a lot like the television remote control—hard to find just when you want it most. ____ But there are ways to increase your willpower.

    *Don't try to change your economic situation, win a promotion or lose weight all on the same morning. Set one clear, specific goal and make a realistic plan to achieve. Extra willpower sometimes requires extra energy, so don't spread yourself too thin. ____

    *Whatever your goal is, don't expect to achieve it overnight,____If you are trying to kick a coffee habit, start by replacing your morning cup of coffee with a glass of water, instead of promising never to drink coffee again. Congratulate yourself on the small achievements. These successes help your willpower grow.

    *____ Ask friends, family or colleagues for assistance and tell them exactly how they can help. If your credit card bills have risen, for instance, let friends know that you are cutting back on expenses. Suggest having a common dinner instead of meeting at an expensive restaurant. Find a support group or organization related to your goal and attend their meetings. You can get valuable advice, understanding and information.

    *If possible, change your environment to encourage positive behavior. Want to get in shape? Keep an extra set of workout clothes in your office as a reminder to stop by the gym on the way home. Quitting smoking?____