Which of the following starts to break down as soon as they enter the mouth?
carbohydrates
proteins
fats
nucleic acids

The answer is carbohydrates.

Food  in  the mouth is moistened and lubricated  with saliva. Saliva contains an enzyme called salivary amylase. 

Salivary amylase is an enzyme that breaks down complex carbohydrates molecules into simple or smaller carbohydrate molecules  called maltose. The enzyme acts on carbohydrates in the mouth, initiating the process of digestion.

Salivary amylase  is made by the salivary glands in the mouth. Maltose is later converted to glucose in the small intestine by another type of amylase called pancreatic amylase.  

easy

explanation:

the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.