In which sentence is the form 'fast' used as an adverb?

The sentence where 'fast' is used as an adverb is the one where it modifies a verb, specifically describing how the cars moved. In this context, 'fast' indicates the manner of the movement, which is a defining trait of adverbs.

In the other sentences, 'fast' serves different grammatical roles. In the first two sentences, it describes a noun (cars and pitch, respectively), indicating their characteristics rather than modifying a verb. In the third sentence, it functions as an adjective describing the state of the clock in relation to time. Therefore, the identification of 'fast' as an adverb in the specified sentence highlights its function in conveying how the action occurs, which is essential for understanding its grammatical use.