Crows will pick up a clam, fly up in the air with it, and then drop it on the rocks to open it. If one were to attempt to assess an ultimate cause for this, what data might one collect?

A) levels of activity in different regions of the brain
B) hormone levels in the brain of the crow
C) comparison of the success of crows that had normal sight and crows whose eyes were covered
D) seeing whether the crows that were most successful in breaking open the clams had more offspring