erza is training for a track race. He starts by sprinting 100 yards. he gradually increases his distance, adding 5 yards a day for 21 days. Which explicit formula models this situation? A. an=100 + (n-1)21 B. an=100 + (n-1)5 C. an=5 + (n-1)100 D. an=21 + (n-1)5