I agree that volcanic activity would be considered a natural cause, but answer choice B never claims that it isn't. Note that B says "meteorite craters." It has nothing to do with volcanoes. It basically states that we have no known explanation for how eight meteors could strike the earth at different times and form a straight line of craters. Remember that our goal is to support the theory that volcanic activity formed these craters. B casts further doubt on the meteor theory, which strengthens the theory that volcanic activity was responsible.
You treated answer choice B as if it said "craters" in general, even though it was specifically referring to "meteorite craters." Make sure to consider the specific wording of each answer choice.
