"That's…"
Initially, Celeste attempted to argue, to counter the points Theo brought up, to defend the validity of the book that she used to such great effect in her own cultivation…
But after killing her initial desire to just respond right away on nothing but her instinct alone and taking some time to think about the points Theo brought, she ended up having no other choice but to actually take her time to think it all through.
Because no matter how hard she wanted to deny it, what Theo said was exactly the same as the very same answer that laid the foundation of Celeste's interest towards him, all the way back during the lesson he was supposed to silently follow.
It was a statement that appeared so extremely obvious, while somehow staying elusive enough for Celeste and the majority of this academy's staff to never realize it.