very very unnutrition but can some one explaine it

I've seen this before, it has something to do with all the possible answers being multiples of nine, and all the multiples of nine have the same symbol.

My son tried to tried to teach that to me, but i didn't master it--math is not my strong suit.  He is a math-guy.  In the 7th grade, he finsihed the 7th grade math book, the 8th grad math book, and started high school algebra.  He took the algebra plus high school scince in the 8th grade--and played on the foodball team (both ways, noseguard and center) played basketball, and baseball.  Entered high school with 2 credits towards grandaution, got football scholarships from one coast to another (first team All State nosegaurd his Junior Year) and accepted full football scholarhip to Brigham Young.  Got most of his sceince classes there, but finished out at SFA and has a degree in Marine Biology and is in the education department at Texas State Aquarium in Corpus....just a little bragging on my kiddo.

it is amazing how some kids are bent toward math, others to music etc...you wonder what little thing in life sent them that way..if he keeps at the math, you can be sure he has a good future...and I have had various explanations  of thie puzzle but still don;t get it