The research for explanations in the many post-mortems of the crisis has meant blame has been spread far and wide. Governments, central banks and auditors have all been in the frame. The role of bank directors and management and their widely published failures have been extensively picked over and examined in reports, inquiries and commentaries. 

这是雅思真题中的一段话的相邻的几句。求指导为什么都用的是完成时，用法分别都是什么呢？

谢谢回答