What is the AGI limit above which each the following taxpayers would not be eligible to receive a credit for the elderly or the disabled?
a)    A single taxpayer eligible for the credit who receives $1,000 of nontaxable social security benefits.
b)    Taxpayers filing a joint return for which one taxpayer is eligible for the credit and the taxpayers have received no taxable social security benefits.
c)    Taxpayers filing a joint return and both are eligible for the credit and received $3,000 of nontaxable social security benefits.