为什么不用has 时间作主语不是用单数吗?The last thirty years have seen the greatest number of

为什么不用has 时间作主语不是用单数吗?The last thirty years have seen the greatest number of
为什么不用has 时间作主语不是用单数吗?“The last thirty years (have) seen the greatest number of laws stopping out rights and progress,until today we have reached a stage where we have almost no rights at all.”