单项选择题
A. the language used by deaf and mute people B. the language used by the retarded C. verbal communication D. body signs
A．it has teachers from various regions in the world B．it has students from various regions in the world C．it has teachers of different races D．it has a powerfully positive impact on student achievement
A．a highly controversial area B．a well-established science C．not associated with terrorism D．not a part of scientific study