A、How to measure metals. B、A standard unit for measuring weight. C、The price of precious metals. D、How to use the metric system.

W: Dr. Thomas? This is Kate Bradley from the daily news. I’d like to ask you some questions about the new official standard weight that you purchased.
M: I’d be happy to help you. What would you like to know?
W: First of all, how was the standard weight used?
M: Well, the people in our department use it to check the scales all over the country. The department of weights and measures, we are a government agency. It’s our responsibility to see that all the scales measure a kilogram accurately so this is the way we use to adjust the scales.
W: How did you check the scales before?
M: We have an old standard weight that we used to use. It had to be replaced because it was imprecise. You see it was made of poor quality metal. It absorbed too much moisture.
W: Oh. So when the weather was humid it weighed more and when it was dry it weighed less.
M: Exactly. And that variation can affect the standards of the whole country. So our department had the new weight made out of higher quality metal.
W: How much did it cost?
M: About 45,000 dollars.
W: 45,000 dollars? For a one-kilogram weight? That’s more expensive than gold. Is it really worth that much?
M: I’m sure it is. Industries depend on our government agency to monitor the accuracy of scales so that when they buy and sell their products there is one standard. Think of the drag industry, fur example, these companies rely on high accuracy scales to manufacture and package medicine.
What is the main subject of the conversation?