The sentence in which the underlined infinitive phrase functions as a subject is the last one - to quickly move large groups of people is the purpose of public transportation.
<em>To quickly move </em>is the infinitive phrase, and it is a subject.
3 maybe if no 4
3 I think so
<h3>Yes really easy and I know you don't need an answer right?</h3>
<span>deepen a poem's meaning</span>
