Fact 1: Adding salt to a solid decreases the freezing point of that solid.

Fact 2: For example, salt is NaCl.

Given the two facts above, answer the question "what can be added to a solid to decrease its freezing point?" with the following options: 
- oxygen
 - ice
 - an ion
 - NaCl
 - potash
 - Acids
 - sugar
 - h2o


 

    NaCl     