why the answer is D not A? Why accrual?

Question 16 A Sharon's financial year ended on 31 December 20x2. She incurs the same telephone expense each month/ The last invoice paid for telephone calls was $3,600. This invoice covered the three months to 31 October 20x2. What adjustment is required when preparing the accounts for the year to 3'1 December 20x2? B AO A B A prepayment of $1,200 V A prepayment of $2,400 An accrual of $1,200 An accrual of $2,400 D