The decision of whether to save Dusk or Luo Lan did not actually matter.
Ren Xiaosu had said humans could live on equal terms with artificial intelligence, so Zero used this question to tell Ren Xiaosu, "Actually, to you, Dusk and Luo Lan are already not equal beings."
In fact, Zero was not expecting to get any answers at all. It only wanted Ren Xiaosu to reflect on the answer he had in mind.
If one day, humans discovered that the artificial intelligence was sapient and had the ability to threaten humans, how would they view it?
But Ren Xiaosu actually did not want to know what the future held. He was only concerned about one thing now, and that was he might not have any more time.
Zero pursued him for a long time using Dusk before suddenly stopping to talk with him.
What worried Ren Xiaosu the most was that Zero was fearless when chatting with him. It was no longer worried he would rush to the Central Plains.