Which one of the following statements regarding the polarity of the Earth's magnetic field is best supported by information in the passage? A. Most, but not all, geophysicists agree that the Earth's magnetic field may exist in two distinct polarity states. B. Changes in the polarity of the Earth's magnetic field have occurred more often in the recent past that in the distant past. C. Heat transfer would cause reversals of the polarity of the Earth's magnetic field to occur more quickly than would asteroid impact. D. Geophysicists' understanding of the reversal of the Earth's magnetic field has increased significantly since the introduction of the heat-transfer hypothesis. E. Friction near the boundary of the inner and outer cores brings on reversal of the polarity of the geomagnetic field.