Determine the amount of the child tax credit in each of the following cases:

\begin{tabular}{|l|l|} \hline & Child Tax Credit Allowed \\ \hline a. A single parent with modified AGI of $215,100 and one child age 4 . \\ \hline b. A single parent with modified AGI of $79,700 and three children ages 7,9 , and 12 . \\ \hline c. A married couple, filing jointly, with modified AGI of $409,633 and two children age 14 and 16 . \\ \hline d. A married couple, filing jointly, with modified AGI of $133,755 and one child, age 13. \\ \hline \end{tabular}