The passage suggests which of the following about changes in the earth's magnetic alignment

A． They are infrequent events, whose causes are yet to be conclusively established.
B． They occur infrequently but at regular intervals, more often through the diffusion process than "field regeneration".
C． They occur regularly, more often through the "field regeneration" process than through diffusion.
D． They occur frequently but irregularly, because "field regeneration" is more often the cause.
E． They occur irregularly, largely because they are dependent on the rare large changes in the flow of the earth's core.