Well if you mean what does the controlled part of a scientific investigation, it is the part of the investigation that will be your control or your base, untouched, original, data and such.
I believe the answer is C.
In seafloor spreading, new oceanic crust is added at the mid-ocean ridge, and, as intrusion continues, seafloor moves toward the edge of the oceanic plate, where it is ultimately pulled back down into the magma below. ... It has deep trenches and high ridges.
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