当你深深呼气时，胸部收缩。

When you exhale deeply, your chest ___contracts.

心脏通过收缩和舒张来推动血液循环。

The heart ___contracts and ___relaxes to pump blood through the body.

气球在冷空气中会收缩。

The balloon ___contracts in cold air.

市政府与建筑公司签订了为期三年的道路维修合同。

The city government ___contracts with a construction company for three years of road maintenance.

她在学校感染了流感。

She ___contracted the flu at school.