For which of the following items might a valuation account be used for the deferred tax item if income is doubtful in future years? Select one: O a. Net Operating Loss Carryforward, but not Unearned Revenue O b. Unearned Revenue, but not Net Operating Loss Carryforward O c. Both Unearned Revenue and Net Operating Loss Carryforward O d. Neither Unearned Revenue nor Net Operating Loss Carryforward