单项选择题
A．All the passengers were killed. B．The plane crashed in the night. C．No more survivors have been found. D．It’s too late to search for survivors.
A．Two ways. B．Three ways. C．Four ways. D．Five ways.
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.