根据句意及汉语提示填空.
__________  __________the answer can help you understand the passage.( 找出)
The car slowed down in time and _________  _________ into the wall. (避免撞击)
He _________  _________his son once a week.(收到……来信)
They _________  _________those problems by themselves at last. (处理)
It’s _______  ________to clean the classroom.(轮到我们)