Trevor takes the subway to and from work. he usually wears his headphones and listens to his music on the subway. he is careful not to bump shoulders with the passengers next to him when he is standing on the crowded train. trevor engages in on the subway.

Unfocused interaction

Explanation:

Unfocused interaction is a behavior in which two or more people, sharing the same space, interact between them with gestures or signals (body language) without any kind of vocal communication.

Even if Trevor is wearing his headphones, listening only to the music (not his sorroundings) he knows, as well as the other passegners of the subway, that it is better no to bump each other shoulders (we all hate that), and, by "tacit agreeement", both Trevor and the passengers around him, will avoid bumping shoulders, even if no one talks about it.

The only possible remaining question may be the cause of it, and i think there are a bunch of reasons for it,  but probably the best one is the force of habit.  

answer; (b) ; /stronger bonds between the nations of africa