Which of the following is true regarding Social Security benefits?

a$.$ The Social Security inclusion formula is the same amount for each filing status.

b$.$ Social Security benefits are always excluded because wages are subject to Social Security tax when earned.

c$.$ Up to $100\%$ of Social Security benefits received may be included in taxable income.

d$.$ Tax$-$exempt interest income must be included in the formula used to determine if Social Security is included in taxable income.