@CatWEax : exactly. And just for this cognitive twist, I loved this sequence. First A-Chan doubt one sec before responding thanks, choose to believe that Mari figure out (but she didn't at this point !) and then Mari asking to her self "hum, why did she thanks me ? Humm, must be... Hoo, there are... actually... together ?". So it all start from a misunderstanding, but my guess is that A-Chan is not really in a misunderstanding, rather in a "not sure" situation, and then she decided to take this opportunity to do as it was abvious that she have a special relation with Chie. It's like a volontaring coming out (not for a gayness, but specially about Chie) by opportunity.
The face A-Chan show when she say thanks, she's so proud of Chie, to be her girlfriend. "Yeah I know, I'm so lucky". So lovely.
This chapter will remain one of my favorite.