Explanation:

- Sales ______ from day to day. The best word to complete this sentence must appropriately describe how sales can change regularly.

- Option 1: fluctuate

- Fluctuate means to vary or change irregularly. Using "fluctuate" fits well as sales can go up and down.

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- Option 2: swing

- Swing means to move back and forth or to vary between extremes. While it's close in meaning, it's less commonly used in the context of day-to-day changes.

- Option 3: revolves

- Revolves means to move in a circular path. This doesn't describe how sales change.

- Option 4: rotates

- Rotates means to turn around an axis. Like "revolves," it doesn't fit the context of changing sales.

Thus, "fluctuate" is the most appropriate word for describing day-to-day sales changes.