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e) The topography of the ocean floors in none too well known, since in great areas the available sounding are hundreds of even thousands of miles apart.

c) However, the floor at the Atlantic is becoming fairly well known as a result of special surveys since 1920.

b) A broad, week-defined ridge-the Mid- Atlantic ridge- runs north and south between Africa and the two Americas.

d) Numerous other major irregularities diversify the Atlantic floor.

a) Closely spaced soundings show that many parts of the oceanic floors are as rugged as mountainous regions.