All of the following are ways in which fatty acids can differ from one another EXCEPT .

a. number of double bonds
b. degree of saturation
c. chain length
d. average number of calories per gram

Average number of calories per gram.

Explanation: Fatty acids refers to any of a class of aliphatic carboxylic acids, of general formula CnH2n+1COOH, that occur combined with glycerol as animal or vegetable oils and fats. Only those with an even number of carbon atoms are normally found in natural fats.

The number of calories gotten per gram in fatty acid is constant. Hence, this factor cannot be used to distinguish between fatty acids..