A．How to care for expensive metal.
B．A standard unit for measuring weight.
C．The value of precious metals.
D．How to monitor the drug industry.

A．To check the accuracy of scales.
B．To calculate the density of other metals.
C．To observe changes in the atmosphere.
D．To measure amounts of rain fall.

A．Someone spilled water on it.
B．Someone broke it
C．It was imprecise.
D．The standard for measuring was changed.