警察奋力阻止人群向前涌。

The police struggled to ___keep the crowd ___back.

请退后——这里发生了事故，需要保持区域畅通。

Please ___keep ___back – there's been an accident and the area needs to be clear.

设置了屏障以阻止观众靠近舞台。

Barriers were put up to ___keep the spectators ___back from the stage.