A period is a horizontal row in the periodic table. Although groups generally have more significant periodic trends, there are regions where horizontal trends are more significant than vertical group trends, such as the f-block, where the lanthanides and actinides form two substantial horizontal series of elements.\n\nBased on this passage, this is a horizontal arrangement of elements in the periodic table? Okie... think carefully first, then make a decision:
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To answer the question, consider the following: A period is a horizontal row in the periodic table. So, the answer is A period.
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