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.  

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

so whats the answer