been是什么时态的关键词

　　There have been many changes in recent years.

　　近几年发生了许多变化。

　　A new system has been devised to control traffic in the city.

　　控制城市交通的新系统已经设计出来。

　　We had to work within the parameters that had already been established.

　　我们必须在已设定的范围内工作。

　　No new SARS cases have been reported in the region.

　　该区没有新增非典型肺炎病例。

　　The information has been placed in the public domain.

　　这资料不受版权保护。