单选题：Periodic realignment of the Earth's magnetic field occurs tw

Periodic realignment of the Earth's magnetic field occurs two ways: the first, "field regeneration", or the creation of new magnetic fields from the ambient geomagnetic field, takes place because magnetic field lines are trapped
Line in effective electrical conductors. The field is trapped in molten iron which
(5)moves in response to the forces imposed upon it and as the core moves, the field lines are stretched and twisted, creating a new magnetic field. The second process is the diffusion of the magnetic fields: in the same way that a drop of dye will soon diffuse throughout a swimming pool, a concentration of magnetic field lines diffuses throughout the planet's outer core. On the large scale of
(10)stars and planets, the field lines are caught up in the fluid motion and distorted, generating a new magnetic field before diffusing away. This rarer type of field reversal is likely caused by unexplained larger changes in the flow in the outer core, though scientists have suggested such fluctuations may simply be extreme examples of fluctuations in the dynamo processes--an El Nino in the weather of
(15)the outer core.
The passage is chiefly concerned with