What to Do About Calcium Deficiency

Blurb: The moment Duanmu Rui opened his eyes, he found himself in an unfamiliar place. It was pitch-black, and he couldn't make out a thing. Before he could figure out what was going on, someone knocked him unconscious again. When he came to, he was forced to move his entire family, and something inexplicable had appeared in his belly. What kind of world is this?! Duanmu Rui was on the verge of going mad. This is an alternate-history ABO world. Don't question the many incongruous and illogical things that happen. One-on-one pairing, no changing the top! Theme: