【Weekly Question】Participate from 9/11 to 9/17, 2017 for +2 wealth； +3 to +10 wealth for correct answers

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To increase the popularity of the chemical analysis section, a \"Question of the Week\" campaign has been launched to foster an active atmosphere for technical exchanges while providing rewards in the form of financial benefits to users. 【Weekly Question】Multiple-choice question: In atomic absorption spectroscopy analysis, in order to prevent backfire, what are the principles that must be followed when turning on and off the gas and the oxidizing gas during the ignition and extinguishing of various flames? ( ) A. Turn on the oxidizing gas first, then turn it off. B. Turn on the gas first, then turn it off. C. Turn on the oxidizing gas later, then turn it off first. D. Turn on the gas later, then turn it off first. Answer: A, D. Reply with your answer in the comments; participating in this question earns +2 wealth points. Those who get all answers correct will receive an additional +3 to +8 wealth points. Those who provide a thorough analysis or explanation of the question or answers, including detailed calculation processes, will receive extra wealth points as a reward. If your answer is correct and your analysis is accurate, it will be marked as the best reply (or the best answer). Feel free to answer the questions.