"You read a book."
Translation:Kamu membaca sebuah buku.
It's like the Queen referring to herself as the Queen, "Ibu membaca ini" to her child. Additionally, the question was only for "you", not ibuk/bapak.
(Technically "they", but they're specific and only used when referring to those two people, in such instances as "mum's reading a book [to me] - or ibu using bapak in place of ia when referring to each other, vice versa - feels rather "young", but it's how it is - also machine parsing)...