Trevor watches as a freight train passes. He counts eight identical black railroad cars, then a red car, followed by several more black cars. Which type of organic compound does the train most closely model?

The black cars represent carbon atoms, and the red car represents an oxygen atom. Since the oxygen is between carbon-containing sections of the train, the pattern models a C–O–C linkage. That is the defining feature of an ether, not a carboxylic acid, which has both oxygen and an –OH group attached to the same carbon.

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