【单选题】There are some earth phenomena you can count on, but the magnetic field, someday is not of them. It fluctuates in strength, drifts from its axis, and every few 100, 000 years undergo a dramatic polari...
A.
Polarity Reversal: A Fantastic Phenomenon of Nature
B.
Measurement of the Earth's Magnetic-Field Intensity
C.
Formation of the Two Poles of the Earth
D.
A New Approach to the Study of Geophysics