(petitioners, married couple, owned an apartment adjacent to their home and, after selling it, attempted to exclude the gain realized on sale from income via I.R.C. Sec. 121; petitioners had leased the apartment to third parties and then to their son and his family; court upheld the determination of IRS that apartment did not satisfy I.R.C. Sec. 121 because petitioners did not live in it an treat it as their personal residence for at least two of the previous five years immediately preceding the sale; court rejected petitioners' claim that their son and his family were part of the "family unit" because petitioners charged son rent and claimed depreciation on apartment; petitioners' son also utilized separate telephone line and mailbox and paid all bills associated with the apartment).