The calculation of the additional Child Tax Credit is first triggered when __________.

a. The taxpayer's modified adjusted gross income (MAGI) exceeds $12,400 in 2016.

b. The taxpayer's modified adjusted gross income (MAGI) is more than 15% over the threshold.

c. More than two children lived with the taxpayer.

d. Due to tax liability limits, the taxpayer did not receive the maximum Child Tax Credit for which they were eligible.