Optional payment in installments up to ten years when interest in closely held business exceeds thirty-five per cent of gross estate.

(b) For purposes of this section “closely held business” means (1) a trade or business carried on as a sole proprietorship, (2) a trade or business carried on as a partnership, provided (A) twenty per cent or more of the total capital interest in such partnership is included in determining the gross estate of the decedent or (B) such partnership had fifteen or fewer partners at the time of the decedent's death, or (3) a trade or business carried on as a corporation, provided (A) twenty per cent or more in value of the voting stock of such corporation is included in determining the gross estate of the decedent or (B) such corporation had fifteen or fewer shareholders. (c) The provisions of this section shall be applicable to the estate of any person whose death occurs on or after July 1, 1985. The estate of any person whose death occurs prior to July 1, 1985, shall be subject to the provisions of this chapter in effect at the time of such person's death.