All of these sentences, especially " C. Gail fell down the steps because she tripped on her untied shoelace" use the word "down" as an adverb since they all use "down" to describe the direction of the fall.
The word "down" is used as an adverb in option B: The ball moved down the street towards the intersection. In this sentence, "down" modifies the verb "moved", indicating the direction of the movement. Both options B and D use "down" as an adverb, but option B is the most clear example for this context.
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