Coffee is a mixture of water, caffeine, and a number of plant chemicals. Different cups of coffee may have different amounts of these chemicals, but the ratio of the chemicals is the same throughout any given cup of coffee. Coffee is

A homogeneous mixture has the same composition throughout. The question says the ratio of chemicals is the same throughout any given cup of coffee, so each part of that cup is uniform in composition. Therefore, coffee is homogeneous; different cups may vary without changing that classification.

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