Questions 19 to 21 are based on the conversation you have just heard.

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.

A．Because the shape of snow crystal(晶体) is still a mystery for experts.
B．Because the snowfall is a blessing for human beings.
C．Because experts have different ideas about the formation of snowflakes.
D．Because it is not easy to judge the time or place of snowfall.