Which of the following statements about Social Security income and taxation is true?

a$.$ Tax$-$exempt interest from municipal bonds must be included when calculating the Social Security inclusion amount.

b$.$ The formula for determining the amount of Social Security income to be taxed is the same for every filing status.

c$.$ After age $65,100\%$ of an individual's Social Security income is taxed.

d$.$ None of a person's Social Security benefits is taxable because they paid the Social Security tax when they were earning.