The word "flip" (last sentence, Paragraph two) most probably means ______.

A．is likely to direct further research in the inner physics of the earth
B．has successfully solved the mystery of polarity reversals
C．is certain to help predict external disasters
D．has caused great confusion among the world's geophysicists

A．its identification of the origin of the earth's magnetic field
B．the way the earth's magnetic intensity is measured
C．its explanation of the shift in the earth's polarity
D．the way the earth's fluctuation rhythm is defined