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C) period

Explanation:

Groups are columns on the periodic table (with the same number of electrons on the outer shell of the elements), whereas periods are rows (increasing in atomic number by 1 - 5, 6, 7, 8, 9 etc.)

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Explanation:

The balance of substances found in the air, as well as food (for plants, this is sun, and for animals, this is plants or other animals) and water. No matter the climate (dry and hot, or wet and temperate, for example), food and water are essential for life.
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