If you guess wrong, you get a small negative score.
If you guess right, you get a positive score.
The negative scores and the positive scores cancel, so guessers come out to zero in the long run.
Guessing does not improve your grade. You are only wasting your time.
Below is a scoring table for a question with a value of ten points, which has one correct choice in ten total choices.
If you want the complete derivation of the grading formulas
(algebra and simple probability theory), please contact support