in lesson 5 in my book 4 (which corresponds to lesson on the website) there's this sentence: DOUYATTE NIHONGO *WO* JOUZU NI NARIMASUKA. (How do I become skilled in Japanese?) I would expect it to be DOUYATTE NIHONGO *GA* JOUZU NI NARIMASUKA. NARU I know is an intransitive verb, so it can't have a direct object. But WO/ O is the direct object marker. So what am I missing? Or is it a mistake in the book?