当然だよ。
Words
当然
(とうぜん)
natural; as a matter of course; justified; deserved; reasonable
だ
(だ)
be; is; indicates past or completed action; indicates light imperative
よ
(よ)
indicates certainty, emphasis, contempt, request, etc.; used when calling out to someone; used to catch one's breath or get someone's attention; yo!