What does the prefix ob- mean in the word obstructive?

Which sentences from part 1 of the call of the wild would best serve as evidence of the theme "sometimes survival depends on knowing when not to fight"? buck had accepted the rope with quiet dignity. to be sure, it was an unwonted performance: but he had learned to trust in men he knew, and to give them credit for a wisdom that outreached his own. he had merely intimated his displeasure, in his pride believing that to intimate was to command. but to his surprise the rope tightened around his neck, shutting off his breath. he saw, once for all, that he stood no chance against a man with a club. he had learned the lesson, and in all his after life he never forgot it. that had given them an unfair advantage; but now that it was off, he would show them. they would never get another rope around his neck.