In Grant Recounts the Surrender at Appomattox, how does the information about Grant

In "Grant Recounts the Surrender at Appomattox," how does the information about Grant's and Lee's shared history in the Mexican War build on another central idea of the text?
A
It explains that the two men are accustomed to talking with each other and do so easily.
B.
It shows that the respect the two men have for each other predates the Civil War.
С.
It sets the stage for the animosity that would arise later between the two men.
D.
It provides an illustration of the way great friends can quickly become enemies.