10Points!Please Help!

Which words in the sentence make up the adverb phrase? Which words do the adverb phrase modify?

A herd of elephants can cover 50 miles a day during the summer.

A.can cover; modifies A herd of elephants
B.during the summer; modifies can cover
C.during the summer; modifies 50 miles a day
D.A herd of elephants; modifies can cover

I've narrowed it down to B and C