Problem description:
All the black people I have met have good teeth.
I'm old, my teeth are white, and I haven't fallen out yet.
However, many people in our country don't talk about getting old and losing their teeth. By the age of 50.60, their teeth will become black, yellow and yellow.
Why?
Analysis:
Black skin can absorb enough ultraviolet rays to convert sterols into vitamins. It is more conducive to the absorption of calcium, and teeth have enough calcium sources.