Marika is training for a track race. She starts by sprinting 100 yards. She gradually increases her distance, adding 4 yards a day for 21 days. Which explicit formula models this situation? A. an = 21+ (n-1)4 = OB. an 100+ (n-1)4 C. an = 4+ (n-1)100 D. an 100+ (n-1)21