To answer the question, consider the following: A period is a horizontal row in the periodic table.is the reasoning for what question and answer pair?

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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Q: 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.

Based on this passage, this is a horizontal arrangement of elements in the periodic table?
A: A period