产品定型、成本下降、利润水平高，但行业增长速度减慢、竞争日趋激烈、企业进入门槛高，具备以上特点的时期属于产品生命周期中的( )。

A．导入期

B．成长期

C．成熟期

D．衰退期

—Can you tell me_____?        

—Sure. Two years ago.      [ ]

A. when did they build the new teaching building        

B. when they build the new teaching building        

C. when they built the new teaching building        

D. when they are building the new teaching building