Sofia put a bat house in a tree in her yard. The bats ate mosquitos in Sofia's yard at night and roosted in the house during the day. Droppings from the bat house nourished the tree. What kind of relationship exists between Sofia, the bats, and the tree?

The questions have been asked in several important state and regional science exams in the United States. For example: TAKS-Texas Assessment of Knowledge and Skills, New York State Educational Department Regents exams, California Standards Test, etc.