To determine the best version of the underlined portion, I would need to see the full context of sentence 13 as well as how the sentence fits into the overall passage. Each option has a different grammatical structure and may convey a different meaning.
- "concern" (Answer B): This is a simple noun and could suggest that there is a feeling of worry.
- "have concerned" (Answer C): This is a present perfect verb form and implies ongoing action or relevance to the present.
- "are concerned" (Answer D): This is a present continuous verb form that indicates a current state of worry.
Without the specific context of sentence 13, it's not possible to definitively select the best option. Could you please provide the complete sentence?