What are the possible disadvantages of evaluating the allowance for doubtful accounts by reviewing individual non-current balances?

What are the possible disadvantages of evaluating the allowance for doubtful accounts by reviewing individual non-current balances?

a. Current accounts receivable may be ignored.
b. It is difficult to compare results of the current and prior years if such an unstructured approach is                   used.
c. Both a and b.
d. None of the above.