You are presented with the question "what can be added to a solid to decrease its freezing point?" and the following answer choices: 
- oxygen
 - ice
 - an ion
 - NaCl
 - potash
 - Acids
 - sugar
 - h2o

Now knowing that adding salt to a solid decreases the freezing point of that solid and for example, salt is NaCl, choose the best answer.


 

    NaCl     