What is labeled A in this picture? 
Definitions:
Protein
Essential molecules made up of amino acids that support muscle growth, repair, and overall body function.
Molecules
The smallest unit of a chemical compound that can take part in a chemical reaction, consisting of two or more atoms bonded together.
Energy Density
The number of calories in a specific weight of food, indicating how energy-dense food is. Foods with high energy density have more calories per gram, while those with low energy density have fewer calories.
Nutrient Density
A measure of the nutrients provided by a food relative to its calorie content, emphasizing foods that are rich in vitamins, minerals, and other beneficial components with fewer calories.
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