Hmm... Another riddle...?

[ComputerNerd]

Complete the sequence.

o, t, t, f, f, s, s, e, n, _, _

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First to answer correctly gets 10 points. I have another riddle still open for answers!
We have a winner! Its... waia2000!

The answer was: t, e.

Why?
(o)ne, (t)wo, (t)hree, (f)our.... .... ine, (T)EN, (E)LEVEN.

 

[Lily]

NO! waia2000 got it before me D:

But whatever, I'll say it anyway. t, e.

 

[waia2000]

t, e
the numbers one through eleven