时态Nothing could have been more annoying

问题描述:

时态Nothing could have been more annoying
上下文:I was busy mixing butter and flour and my hands were soon covered with sticky pastry .At exactly that moment ,the telephone rang .Nothing could have been more annoying
最后一句什么时态?
前面的就不要管了,最后一句话,里面有个have been .我是知道have been 是一般现在完成时,但前面有个could ,所以我看不懂了

could have been 表虚拟语气这是什么时态呢?过去完成时