TOEFL Listening: ETS-TOEFL听力机经 - S8Y844FC6P8A389FP$

What does the professor imply about researches who attempt to study natural occurrences of ball lightning? A. They should not trust eyewitness accounts B. They will probably confirm that it is an optical illusion C. They need to have a great deal of patience D. They have correctly predicted its occurrence only a few times