In group annuity contracts there shall be a provision specifying the document or documents which shall constitute the entire contract between parties. The document or documents so specified shall be only (a) the contract, (b) the contract together with the application of the holder of which a copy is attached thereto, or (c) the contract together with the application of the holder of which a copy is attached thereto, and the individual applications of annuitants on file with the insurer and referred to therein.