Which of the following did Mexico and the Republic of Texas continue to fight about in the 1840s?

the location of the US-Mexico border the practice of slavery in Texas

the location of the Mexico-Texas border

the practice of slavery in Mexico

During the Mexican-American War, the US declared that US territory bordered the Rio Grande, using the 1836 Treaties of Velasco as proof. Mexico disagreed and stated that their border was actually at the Nueces River, about 150 miles north of the Rio Grande. When the US went to the Rio Grande, that's when the actual fighting started because Mexico felt as if they were invading on their land.

3rd option just did this