A、At the beginning. B、In the middle. C、Near the end. D、During the final exam. A题目问的是对话发生在学期的什么时候。由定位句可知，实验是为了补充无机化学课，这节课刚开始熟悉内容，

Good afternoon. My name is Mary Raffety. For the next eleven weeks, (26) I will be your lab instructor. The lab experiences you will have are designed to complement your work in Dr. Kaplan’s inorganic chemistry course. Today’s experiment is purposely a short one, (27) it will help you become familiar with the lab setup and equipment.
    As your lab instructor, it is my duty to assist you in setting up your experiments and understanding your results. I will also grade your lab notebooks. (28) But I have an even more basic responsibility: your physical safety. I will insist on proper precautions, such as wearing protective goggles at all times. I also expect you to use common sense: (29) don’t wear long scarves that might catch fire; don’t smoke; don’t taste unknown substances.
    (30) Let me reinforce this point with a story. Isaac Newton, perhaps the greatest scientist of all ages, lived in a period when the toxic effects of chemicals were less understood than today. He routinely sniffed fumes, tasted chemicals, and used open containers for heating substances. In the early 1690s, he suffered through a period of insomnia, depression and mental instability. Though his biographers linked this situation to problems in his personal life, researchers now think it was a consequence of his lab procedures; they found abnormally high concentrations of lead, mercury and other heavy metals in preserved specimens of his hair.
    Consequently, we must learn from the past and put safety first.
26．Who is the speaker?
27．At what point in the semester does this talk take place?
28．Why is the speaker addressing the students?
29．Which of the following are the students told not to wear?
30．Why does the speaker tell the story about Newton?