In which sentence is "fast" used as an adverb?

In the context of the question, "fast" is used as an adverb in the sentence where it describes how the cars moved. Adverbs modify verbs, adjectives, or other adverbs, and in this case, "fast" modifies the verb "moved," indicating the manner in which the cars are moving. This usage emphasizes the speed of the cars.

In the other sentences, "fast" serves different grammatical functions. In the first sentence, "fast" is an adjective describing the clock’s timekeeping. The second sentence is where "fast" functions as an adverb describing how the cars moved. The third sentence uses "fast" as an adjective to describe the noun "cars," indicating that they are designed for speed. Finally, in the last sentence, "fast" also acts as an adjective modifying "pitch," referring to the type of pitch being thrown. Thus, the use of "fast" as an adverb effectively conveys the action of moving quickly.