液化气体的毒害性体现在（）。
A．毒性气体对人体的危害
B．压力过高对人体的危害
C．空气中含量过高使人员窒息
D．A和C
参考答案：D
当管壁厚度小于3.5mm时，其管道焊接的坡口形式是( )。
A．I形坡口 B．V形坡口 C．U形坡口 D．Y形坡口
The quantity of labor that a profit maximizing firm will employ, holding other input factors constant, is the level at which:（）
A. the marginal revenue product of labor is equal to the wage rate.
B. the marginal product of labor is equal to the marginal cost of labor.
C. one more unit of labor would cost less than the value of its additional output.
