I need help with this question it's "Traveling" by Grace Paley click to see the question... Please Do Not Steal The Points Or I Will Get You Banned :) What is “the invisible absolute racial border” the author mentions in paragraph 13?

My mother and sister were traveling south. The year was 1927. They had begun their journey in New York. They were going to visit my brother, who was studying in the South Medical College of Virginia. Their bus was an express and had stopped only in Philadelphia, Wilmington, and now Washington. Here, the darker people who had gotten on in Philadelphia or New York rose from their seats, put their bags and boxes together, and moved to the back of the bus. People who boarded in Washington knew where to seat themselves. My mother had heard that something like this would happen. My sister had heard of it, too. They had not lived in it. This reorganization of passengers by colour happened in silence. My mother and sister remained in their seats, which were about three-quarters of the way back.

Explanation:

which statement best expresses the theme of this short story the value of being confused