What Causes Magnetic Stripes on the Seafloor,such as Those Shown

What causes magnetic stripes on the seafloor,such as those shown here?

A) Volcanic rocks in oceanic crust are covered by a variable thickness of sediment.
B) Some oceanic crust consists of volcanic rocks and some is composed of sediment.
C) Sometimes Earth's magnetic field points north and sometimes it points south.
D) Some volcanic rocks are formed in normal ways and others have pillows.