单项选择题
A．on B．to C．at D．for
A．present smog standards should be made stricter B．the standard established by the EPA is effective C．some areas fail to meet the federal standard at times D．poor air quality is a major problem nationwide
A．parallels a social tendency of being informal B．worries students as well as professors C．is taken as trivial by employers D．is ignored in all business concerned sciences