Fact 1: High pressure systems cause clear weather.

Fact 2: Weather Clear weather usually means good visibility.

Given the two facts above, answer the question "What causes good visibility?" with the following options: 
- permineralization
 - focusing a lens
 - high pressure systems
 - flashlights
 - peachleaf willow
 - the Great Barrier Reef
 - an electron microscope
 - windpower operated machines


 

   high pressure systems      