[
  "Work step by step: find how many are rotten, how many original remain, then the new total, and finally the fraction.",
  "Rotten = 5/8 x 72 = 45. Original not rotten = 72 - 45 = 27. New total = 27 + 18 = 45. Fraction = 27/45 = 3/5.",
  "3/5 of the oranges now in the refrigerator are from the original batch. Answer: 3/5 (option B)."
]