A．A standard unit for measuring weight．
B．How to care for precious metals．
C．The value of precious metals．
D．Using the metric system．

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

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