C. the effects of sea floor spreading
Explanation:
The effect of sea floor spreading is not a subtopic in this paragraph. There was no mention of even the term sea floor spreading in this context.
The paragraph was all about plate tectonics and its effect. Also, it specifically chose to describe movement along the San Andreas fault and the likely consequences of such movement.
- The topic sentence lies in the first line where plate tectonic theory was mentioned.
- The movement along the San Andreas fault is result of plate tectonics.
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