单项选择题
A．Planes and motorcars have taken the place of trains. B．Oil is expensive today. C．Trains are slow. D．Railways lose money.
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.