(1) There has been a recent disappearance of honey bees, referred to as Colony Collapse Disorder. (2) Many theories have been advanced to explain this disappearance. (3) One theory blames pesticides, which cause an insect to become lost and confused. (4) Another explanation is that stress is lowering bees’ immune systems. (5) Still other experts blame IAVP, a parasitic virus that kills bees. (6) All experts agree that more research is needed to develop a single theory. Which is the topic sentence of this paragraph?