21. For the sake of logic and coherence, Sentence 5 should be placed:

Answer and Explanation

Your Answer is

Correct Answer is D

Explanation

The fifth sentence "signs are installed on the entire lawn to threaten all kinds of extreme behavior and prevent intruders." There is a but no one is ever around to ensure the threats in the fourth sentence above, judging according to the meaning , the fifth sentence should be placed before the fourth sentence, item D is correct.