Fact 1: Many plants respond to the days growing shorter in the fall by going dormant.

Fact 2: Plants are dormant in the winter.

Given the two facts above, answer the question "What causes plants to sleep?" with the following options: 
- winter
 - heat
 - lmestone
 - sweating
 - photons
 - Sun
 - Light
 - air cools


 

 winter        