Abbott (an S Corporation), Bell, Costello, and Dilbertson form Best Partnership, an equally-shared partnership. Each partner but Abbott has a December 31 tax year-end; Abbott's is June 30. The partners would like to choose a partnership tax year other than one ending December 31, but do not have a valid business purpose for doing so. Which of the following is an acceptable tax year-end they may choose for Best Partnership? December 31 September 30 June 30