How does the earth leave a permanent magnetic record on the ocean floor?

Sea floor striping is a fascinating subject.  As the sea floor spreads outward from the central zone, the rock rapidly cools and congeals, preserving a record of Earth's magnetic orientation within ferromagnetic particles (tiny bits of iron) entrapped in the rock.

Pole reversals are recorded in this rock.  I would not claim the record was permanent, however, in that along plate margins the sea floor is subducted into the mantle, where the record is destroyed.

Answer decreased soil salinity;

Why you put last one a? i think is b.