What is the purpose of the lines that are underlined?
These are answers to topic questions, and the
para saphs after them explain the answers.
These are summary points so that the reader could
just read these lines and understand the passage.
These are conclusions that summarize all of the
information in the paragraphs before them.
These are headings that give titles to the topics being
discussed in the paragraphs after them.

a mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. this uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.a geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged.

the first settlers in north america were frim europe.

hope it u