The children went home from the grammar school, their lessons __________ for the day.

A．finishing)

B．finished

C．had finished

D．were finished

《房地产估价规范》条文说明中给出了钢结构房屋的经济耐用年限的参考值如下：生产用房70年，受腐蚀的生产用房50年，非生产用房( )年。

A．50
B．60
C．70
D．80