Answer:
B. People are cutting other classes to sit in
Step-by-step explanation:
The setting of Todd Strasser's novel "The Wave" is Gordon High School where a history teacher, Ben Ross conducts an experiment (creates a student organization called "The wave") in order to explain his students how and why Nazis rose to power in pre-WW2 Germany.
He uses his history class to establish the organization, make the rules, logo, moto, salute etc.
Students seem to accept this as it gives them sense of belonging to a group or a community; they feel stronger, more special and privileged.
As the organization starts to spread, more and more students attend Mr. Ross's history class, many of which weren't taking this class at all and started skipping their regular classes, which can be seen from Chapter 9 of the novel.