...I did kill some of my brain cells to solve this one...First thing I tried to find out the common thing between all the answers...and indeed, all the answers were multiple of number "9"...So, whoever created this, was very clever to use same sign for all the multiples of "9". Signs do change randomly, however all the multiples of "9"(9,18,27,36...) always have a same sign...

..anyways, I was really amazed to find out this new math info...aren't u guys?

-Americano!