This was a new word - i hovered over it to see the meaning and it came up "For or Since" I just picked one and put "For" and got marked wrong.
If that is not an acceptable answer it shouldn't come up when you hover over the word.
Both are correct meanings in the right contexts :
- Dyn ni'n byw yma ers y nawdegau - We have lived here since the 90s.
- Ro'n i'n aros am y bws 'na ers oesoedd - I was waiting for that bus for ages.
'For' is now added as a correct answer - thank you for the comment.