DAT Destroyer Question: Identifying an unkown compound

[tararene28]

An unknown compound needed to be identified. Studies revealed that it reacted with oxygen and fluorine to yield Y2O3 and YF3. Which element listed below could be element Y?
a. As
b. C
c. Mg
d. Ar
e. Hg

the correct answer is As, but I'm not quite understanding the reasoning they give you in the DAT destroyer solutions. I've looked through my notes from Chad's videos and can't find where I could learn more about this type of question. Help?
DAT Destroyer Gen Chem question 58.

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[Alpha Centauri]

Arsensic has an oxidation number of +3. For example fluorine has a -1 charge, making there be 3 Fluorines would mean that the unknown compound would have a +3 charge to make the compound neutral