Answer: We sprayed our yard to eliminate all the mosquitoes. In this sentence, the infinitive is used as an adverb.
Step-by-step explanation:
The sentence which contains an infinitive phrase is the last sentence: We sprayed our yard to eliminate all the mosquitoes.
The infinitives are verbals - verb forms which function as other parts of speech (nouns, adjectives or adverbs). Apart from the infinitives, gerunds and participles are also considered verbals.
In this particular sentence, the infinitive is used as an adverb. Adverb infinitives are usually found either at the beginning or the end of a sentence. They modify verbs and answer the question 'why.' In this particular case, adverbial infinitive explains why people sprayed their yard.