-Mum,where is my lunch pack?-Just-----it was.a.where b.what

-Mum,where is my lunch pack?-Just_____it was.a.where b.what c.which d.how
为什麼是a呢?where不是状语从句吗,was後面不是少了宾语吗?T T.为什麼噢为什麼.