Which of the following is true of the exclusion of gain under & 121? a. The taxpayer (or spouse) must be 55 years of age or older on the date of sale of the principal residence in order to apply the exclusion. b. The residence must have been used as a principal residence by the owner/ taxpayer (or spouse) for at least two years within the five-year period end- ing on the date of sale c. The exclusion is elective but may not be revoked. d. The exclusion may be used only once by a taxpayer