I believe the linking verb is “will”. It’s linking the subject “the United States of America” and the predicate “be 300 years old!”
Answer:
lettuce farm
economical challenges
she might not be able to concentrate
B, because running is a verb and none of the other sentences have verbs, verbs are action words.
Names a particular person, place, thing, or idea. They are words which name specific people, organisations or places. They always start with a capital letter. E.g. the Civil War, Monday, or Potter.