(A) describes the primary purpose. The author wanted to describe the impact of a “new tactic” (sit-ins) on the civil rights movement.

(B) focuses on the events leading up to the first sit-ins. But the passage focused instead on the effects of the first sit-ins, which happened after those sit-ins.

The only technique discussed by the author is sit-ins. If you think that the SNCC and Freedom Rides are different “techniques and tactics” from the original sit-ins, you’re either misinterpreting the passage or the phrase “techniques and tactics.” The SNCC was a civil rights organization – it was not a technique or tactic that anyone used to expose the injustice of racial segregation. And, Freedom Rides were a form of sit-ins. They were the same technique – sitting in places designated “whites only”. Thus, since the passage didn’t discuss any technique or tactic besides sit-ins, the purpose of the passage cannot be to contrast various techniques and tactics.

Even if you think that Freedom Rides were a different technique from sit-ins, the purpose of the passage is not to contrast original sit-ins with Freedom Rides (or any other technique). The purpose is to present the consequences of the original sit-ins.

The author does not try to persuade us about the necessity of sit-ins. Instead, the passage simply described what sit-ins were and how they affected the civil rights movement. The passage never said that they were necessary for the civil rights movement to be successful.

The author does discuss how the original sit-ins led to later successes. But, the overall purpose of the passage is focused on the original sit-ins and their impact. The passage does not discuss any other “earlier successes” of the movement that spurred later activities.