What was the MOST crucial factor in the failure of diplomatic negotiat

What was the MOST crucial factor in the failure of diplomatic negotiations between the United States and Mexico to resolve their bilateral tensions and conflicts in 1846?

A) President Polk insisted, along with many Texans, that the Rio Grande was the legitimate western boundary of Texas.
B) President Polk insisted, along with many Texans, that the Nueces River was the legitimate western boundary of Texas.
C) The hardline, extremist Mexican President Herrera refused to negotiate in good faith with the United States over a compromise boundary for Texas.
D) President Polk continued to support domestic revolts to overthrow the government of Mexican President Herrera.

ANSWER
Answer: A