g are married, both are claiming social security retirement benefits and have the following sources of income: they have $2,000 of muni-bond interest, $34,000 of social security benefits, and $3000 of preferred stock dividends. in addition, both are subject to the minimum distribution rules. last year, none of their social security benefits were taxable. however, this year, $5,000 of their benefits were taxed. why might the taxation of their benefits change from last year to this year?