It was only when I read his novel once again recently ---- I

It was only when I read his novel once again recently ____ I came to realize ..It was only when I read his novel once again recently ____ I came to realize the effect it ___ on the young generationa.until; put b.that; had c.then; made d.did; has选B,为什么?that在句中起什么作用?引导什么从句