Tris gets all up in Christina's business. ("Do you like Uriah. Like, like like him?")
The answer: no. But Christina gets all up in Tris's biz, too: "Where have you been lately? […] With Four? Doing a little… addition? Multiplication?" (24.12)
Tris responds that they've been finding the cosine of the angle between two vectors, thank you very much. Okay, she doesn't say that, but she totally should have.
From there, the conversation switches to genes, as conversations about sex often do. Tris and Christina wonder who "arbitrarily decided that one set of DNA was bad and the other was good" (24.21). Well, the leaders of this government. (Not to mention the leaders of some nasty 20th-century governments, too.)
Tobias comes in and tells Tris that he went to meet Nita last night.
Tris is immediately furious.
Tobias tries to explain: "She just wanted to show me something" (24.43). Oh, Tobias. That is not the right choice of words.
Tobias tells Tris the whole story, and she's not jealous anymore. She's still suspicious of Nita's motivations, though.
To convince Tris otherwise, Tobias decides to bring her when he meets Nita next.