单项选择题
A．At midnight. B．In the morning. C．When the car is turned off. D．When the neighbor’s son leaves for work.
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．How to care for precious metals. B．A standard unit for measuring weight. C．The value of precious metals. D．Using the metric system.