A TOEFL Reading passage discusses the migration patterns of monarch butterflies. The passage states: "Unlike most insects, monarchs undertake multi-generational migrations spanning thousands of miles, a phenomenon that has puzzled scientists for decades." The word "phenomenon" refers to:
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A
A scientific law with a clear explanation
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B
An observable fact or event that is remarkable or not yet fully understood
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C
A migratory route that has been fully mapped
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D
A genetic mutation in the butterfly population