Nguyen thi vinh describes the division between north/south vietnam as "a frontier of？

you didn't provide the passage.

explanation:

seeing as this is your first question on i can see why you may have not known that you need to provide the passage.

without the passage, or even the name of the passage, we can not read the passage, therefore we can not determine when or how the pacing effects the tone.

however, you can either repost this question (providing the passage this time)

or you can comment below what the passage is, and i'll you to the best of my ability!