In $2010$ Casey made a taxable gift of $$6\mathrm{.}3$ million to both Stephanie and Linda $($a total of $$12\mathrm{.}6$ million in taxable gifts$).$

Calculate the amount of gift tax due this year and Casey's unused applicable credit under the following alternatives. $($Refer to Exhibit $25-1$ and Exhibit $25-2.)$

Note: Enter your answers in dollars, not millions of dollars. Leave no answer blank. Enter zero if applicable.

a$.$ This year Casey made a taxable gift of $$1$ million to Stephanie. Casey is not married, and the $2010$ gift was the only other taxable gift he has ever made.

b$.$ This year Casey made a taxable gift of $$16\mathrm{.}3$ million to Stephanie. Casey is not married, and the $2010$ gift was the only other taxable gift he has ever made.

c$.$ This year Casey made a gift worth $$16\mathrm{.}3$ million to Stephanie. Casey married Helen last year, and they live in a common$-$law state. The $2010$ gift was the only other taxable gift Casey or Helen has ever made. Casey and Helen elect to gift$-$split this year.