Fact 1: High pressure systems cause clear weather.

Fact 2: Clear weather is caused by sinking air.

Given the two facts above, answer the question "What causes high pressure systems?" with the following options: 
- rainfall
 - hyperthyroidism
 - heating liquids
 - mechanical
 - chemical changes
 - sinking air
 - fossil fuels
 - our nearest star


 

      sinking air   