做心肺复苏时需要胸外按压。

Chest ___compressions are needed during CPR.

医生建议使用胸部按压来帮助患者恢复呼吸。

The doctor recommended chest ___compressions to help the patient regain breathing.

当火车朝我们驶来时，我们会听到声音频率变高，这是由于声波的压缩效应。

As the train moves towards us, we hear a higher pitch due to ___compressions in the sound waves.