Which of the following is true about the portability of unified transfer tax credit? a. If one of the spouses dies, then any unused applicable exclusion amount will carry over to the other spouse. b. If one of the spouses is left with an excess amount after using the applicable exclusion, then the excess amount will carry over to the other spouse. c. It is a method of reducing gift taxes with the consent of the other spouse, in which the gift can be treated as if each had been given one-half of it. d. It is a method of transferring assets between spouses, either by gift or through estate. e. If one of the spouses retires, then any unused applicable exclusion amount will carry over to the other spouse