已知a、b、c成等比数列，a、x、b和b、y、c都成等差数列，且xy≠0，那么$\frac{a}{x}+\frac{c}{y}$的值为（  ）

安史之乱的时间是（    ）

下列各句中，没有语病的一句是（    ）

【自然灾害与防治】根据题文材料，完成下列要求。

下图为华北平原某地级行政区域多个气象观测站关于大风天气的多年统计资料，该区域的大风天气多由冷空气南下引发形成。

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        Having good etiquette (礼节) at the workplace is very important to be a favorite in an office.However，it’s observed that many people aren’t aware of the workplace etiquette and this creates a very bad impression in the office.So it’s important to know some workplace etiquette tips.

       Among all the workplace etiquette guidelines，the most important is to be punctual to your office.Though going late due to an emergency is okay，habitual late comers are never appreciated in any organization.By arriving at your office on time，you show that you’re aware of your responsibilities and have respect for the organization.In case you feel that you would be late，call the concerned authority and report the matter to him or her.

         Also，a proper knowledge of the workplace email etiquette is a must.In the official emails，you need to mention the subject concisely，while at the same time include all the important details which are to be shared.You should use good and grammatically correct language while writing emails.

         A knowledge of telephone etiquette in the workplace is very important as well.While talking on the telephone，be polite and listen to what they’re saying carefully.Only then should you say what you feel.Speak in a voice which would be heard clearly at the other end.

         The workplace guidelines are important even while you’re dining or celebrating with your co­workers.If you get a call in between，receive it after you’re permitted by the others by saying “excuse me”．Don’t talk loudly while eating.Greet people well and try to make them feel comfortable while being in your company.

        These guidelines will help you become the best employee of a company.All the best!