A:has been introducing
B:is being introduced
C:is introduced
D:was introduced
夏季我国平均气温最低的地形区是____原因是____。
The findings support another study which showed that being happy ______ reduce the risk of heart disease.
南京大屠杀遇难同胞纪念馆进口处镌有“死难者300000”的字样,这样做的主要目的是( )