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 rainfall.

A．someone spilled water on it.
B．someone lost it.
C．it was made of low quality metal.
D．the standard for measuring had changed.

A．It is a small amount to pay for so much precious metal.
B．It is difficult to judge the value of such an object.
C．It is reasonable for an object with such an important function.
D．It is too high for such a light weight.