The government has been under growing pressure to ______ the causes of air pollution as smog frequently smothers the country's large cities．

A．release

B．schedule

C．address

D．maintain

凡在科学研究上有杰出成就的人，不少是在物质条件十分艰苦的情况下，经过顽强刻苦的努力才获得成功的。