84 pts ! and brainliest to first correct answer 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 b. an = 4 + (n – 1)100 c. an = 100 + (n – 1)4 d. an = 100 + (n – 1)21