What Assumption Did Richardson and Verbeek (1986) Make When Testing

What assumption did Richardson and Verbeek (1986) make when testing the prediction of the optimal diet model in northwestern crows?

A) Clams of different sizes represented different food types
B) Individuals have different optimal diets
C) All clams have the same handling time
D) All clams have the same energy value
E) Crows need to learn how to feed on clams