问题
单项选择题
关于用例规约，以下说法正确的是（）。
A、用例规约与用例图表达的信息是一致的
B、用例规约包括基本事件流和备选事件流
C、基本事件流不能表示失败场景
D、备选事件流不能表达成功的场景
答案
参考答案：A
搜题请搜索小程序“爱下问搜题”
关于用例规约，以下说法正确的是（）。
A、用例规约与用例图表达的信息是一致的
B、用例规约包括基本事件流和备选事件流
C、基本事件流不能表示失败场景
D、备选事件流不能表达成功的场景
参考答案：A
Professor Williams lived alone and he was very ．He used to arrive at the university to give a lecture and find that he had forgotten to bring his notes．Or he would lose his spectacles and be unable to see the ．He could never find any chalk to with，and he often forgot the time and would ramble(漫谈)on for hours because he had his watch at home．But the most thing of a11 about him was his ．His overcoat was rarely ，as most of the buttons were ．His shoes were usually untied because he had lost the laces．He must have lost his comb as well because his hair was always standing on．His trousers were by an old tie instead of a belt．He was a chain smoker．He would smoke in class．Cigarette ash was liberally scattered over his waistcoat．
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19世纪末，英国科学家瑞利通过如下两种实验方法制取氮气，从而导致了科学史上某种气体的重大发现．请你参与他的科学发现过程，填写下列实验报告：
实验一：从空气中分离出氮气
| 实 验 步 骤 | 解释或结论 |
| （1）将洁净的空气通过过量的NaOH溶液，再通过浓硫酸 | 通过过量的NaOH溶液的目的是为了除去空气中的______． |
| （2）将处理后的气体通过能与氧气反应的物质，而除尽空气中的氧气． （瑞利提醒你：反应不能生成新气体） | 除尽空气中氧气的化学方程式是______ |
| （3）收集剩余气体，并测定其密度 | 密度为1.2572克/升 |