| A | B |
|---|
| human beings | people |
| essential | necessary |
| carry out | complete |
| masses | large amounts |
| available | obtainable; accessible |
| continuous | without stopping |
| cycle | group of events that happen in the same order again and again |
| atmosphere | air that surrounds the earth |
| vapor | tiny drops of fluid in the air |
| clouds | tiny drops of water that collect in the air |
| usable | able to be used |
| evaporation | the sun heats the water and changes it to water vapor |
| condensation | when cool vapor changes to small droplets of water |
| precipitation | when drops of water in clouds become heavier, they fall back to Earth |
