Crows Feeding on Whelks Drop the Whelks from the Air

Multiple Choice

Crows feeding on whelks drop the whelks from the air until their shell breaks,allowing the crow to eat the mussel inside.What factor(s) is(are) important for crows optimizing this foraging strategy?

A) the force of gravity in each drop
B) the number of drops needed to break the shell and the height per drop
C) the number of drops needed to break the shell
D) the force of gravity in each drop and the height per drop
E) the height per drop