Final answer:
The words in bold are an infinitive in the sentence 'We felt that remaining at the library was a waste of time.'
Step-by-step explanation:
The sentence in which the words in bold are an infinitive is: We felt that remaining at the library was a waste of time.
An infinitive is a verb form that typically begins with the word 'to' and functions as a noun, adjective, or adverb. In this sentence, 'remaining' is an infinitive that functions as a noun, representing the action of staying at the library.
Learn more about identifying infinitives