20 adverb modifying go.
21 Reading a book is a gerund phrase subject of the linking verb is. [It's the only phrase in the sentence].
22 Of the United States is an adjective phrase modifying president.
23 To get. Infinitive It's acting like a noun
24 Participial phrase modifying dog
Answer: Its C
Explanation: Cause in you're question it say "would probably NOT be used as support for a proposition to keep a city's public libraries open seven day's a week" and c is saying fewer and fewer are using the libraries which means c would not be used as support for a proposition.