The abbreviation gal. is for

If johnny ran a mile, where would he be?

Acurious child asked his mother: “mommy, why are some of your hairs turning grey? ” the mother tried to use this occasion to teach her child: “it is because of you, dear. every bad action of yours will turn one of my hairs grey! ” the child replied innocently: “now i know why grandmother has only grey hairs on her head.”

According to the hierarchy of chords, fill in the missing chord in this progression. i - iv - - i?