Answer and Explanation:
This question is about "The outsiders" book written by Susan E. Hiton. The book portrays the lives of young people in street gangs, showing the rivalries and violence between them, in addition to showing the economic difficulties that this environment promotes.
Ponyboy was extremely surprised when Cherry declared that she is a soc. That's because Cherry had had thoughts very similar to Ponyboy's, making him not imagine that she was from a different gang than his. From this form, we can conclude that Ponyboy's surprise is also the result of the understanding that rival gangs think similarly and that they could even be friends.