June made taxable gifts as follows: $400,000 in 1973, $200,000 in 1977, $600,000 in 1985, and $700,000 in 2001. In 2013, June dies leaving a taxable estate of $4,000,000. Junes tax base for applying the unified tax rate schedules (for estate tax purposes) is:

a. $4,000,000. b. $4,500,000. c. $5,300,000. d. $5,500,000. e. None of the above.