When the needs of the service require, the Secretary may direct a selection board, which has been convened under
section 2106 of this title, to recommend for continuation on active duty for terms of not less than two nor more than four years a designated number of officers of the grade of lieutenant who would otherwise be discharged or retired under this section. When so directed, the board shall recommend for continuation on active duty those officers under consideration who are, in the opinion of the board, best qualified for continuation. Each officer so recommended may, with the approval of the Secretary, and notwithstanding subsection (a), be continued on active duty for the term recommended.