Part III. 16-20: When I was a boy living on a ranch in Tennessee, I spent glorious afternoons__(16)__ my best friend. We'd hike into the woods, ride ponies, visit the rodeo arena, and venture into the barn to watch the cowboys work the horses. __(17)__ whenever I heard my dad's whistle—that clear sound __(18)__ the wind and all the other clatter—I'd immediately drop whatever I was doing and head home. The signal was unmistakable, and I knew I __(19)__ by my father. Decades later, I'd still__(20)__ that whistle.
【題組】20.
(A) vitalize
(B) legalize
(C) recognize
(D) polarize