短语填空 1 . He returned home ___________( 如释重负 )when he got the news that he had passed the exam. 2 . No one knows when such a custom first ________________( 形成 ) . 3 . There was a time when a play became highly successful even though most of the experts blamed it ________________( 毫不留情地 ) . 4 . No one knows ____________( 肯定 ) how people first learned to preserve food. 5 . Tom was fired by the company, but it wasn't ______________( 很久以后 ) he found a new job. 6 . Up to now, no one has come up with a convincing explanation of why dinosaurs ________________( 灭绝 ) .