听力原文： Good afternoon. My name is Mary Raffety. For the next eleven weeks, I will be yo

ur lab instructor. The lab experiences you will be having are designed to complement your work in Dr Kaplan's inorganic chemistry course. Today's experiment is purposely a short one; 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. 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: don't wear long scarves that might catch fire; don't smoke; don't taste unknown substances.

Let me reinforce this point with a story. Issac 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.

At what point in the semester does this talk take place？

A．At the beginning.

B．In the middle.

C．Near the end.

D．During the final exam.