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They basically got engaged when He Tian came back on Christmas and gave Mo the rings, since then they have official art wearing the rings like a necklace or in the middle finger. It's chinese tradition to put the ring in the middle finger when you're engaged since the fourth finger is reserved only for marriage

I saw in x that hi tian and little mo got engaged (▰˘◡˘▰) ahhhhhh waiting for the updates