For the following questions, refer to the following passage and illustration. A team of scientists is studying the composition of the sea floor near the underwater mid- Atlantic ridge volcanoes. They noticed that the ocean floor had regular alternating magnetic bands on either side of the ridge. Their observations are shown in the diagram. This effect is related to regular polarity reversals in the north and south magnetic poles of Earth.

With the understanding that solid land masses float on the liquid mantle, the scientists have concluded that the addition of new ocean floor at the mid-Atlantic ridge is the result of
a
seafloor spreading.
b
subduction.
c
earthquakes.
d
mountains.