单项选择题
A．does not disturb existing pressures B．stays behind its disturbance C．does not produce great enough changes in pressure D．stays ahead of its disturbance
A．discover what causes the sonic boom B．find a means of eliminating the sonic boom C．produce the sonic boom at low altitudes D．reduce the sonic boom to tolerable levels
A．protecting children B．assisting shepherds C．helping hunters D．herding cattle