根据短文理解选择正确答案。

      A good way to pass an exam is to work hard every day in a year. You may fail （不及格）in the exam if you are lazy for most of the year and then work hard only a few days before the exam. If you are taking an English exam, do not only learn rules of grammar. Try to read stories in English. Speak in English whenever（无论何时）you can. A few days before the exam you should go to bed early. Do not stay up late at night studying. Before you start the exam, read carefully over the question paper（试卷）, try to understand the exact(确切的) meaning of each question. When you have finished your exam, read over your answers. Correct(改正) the mistakes if there are any, and be sure that you have not missed anything out.

大飞机是国之重器。上世纪八十年代,我国中止大于飞机研制,转为与国外合作组装生产,但在长期合作过程中并没得到核心技术。九十年代后期我国转向自行研制,经过十多年攻坚,大飞机终于在2015年成功下线。材料启示我们,发展战略性产业应（    ）

已知室温时，0.1mol/L某一元酸HA在水中有0.1%发生电离，回答下列问题：

如图，用滑轮组从H=10米深的水中匀速提起底面积为0.04米²、高2米的实心圆柱体，该物体的密度是2.5×10³千克/米³．现在动滑轮挂钩用钢丝绳与该物体相连，已知绕在滑轮上的绳子能承受的最大拉力F为700牛．