Assuming that an AGI limitation does not apply, what amounts of credit for the elderly or the disabled would be permitted in each of the following instances?

\begin{tabular}{|l|l|} \hline & Allowable Credit \\ \hline a. A single taxpayer eligible for the credit who receives $2,100 of nontaxable social security benefits. & \\ \hline b. Taxpayers filing a joint return for which one taxpayer is eligible for the credit and the taxpayers have received no social security benefits. \\ \hline c. Taxpayers filing a joint return, and both are eligible for the credit and received $4,100 of nontaxable social security benefits. & ; \\ \hline \end{tabular}