今有一劲度系数为k的轻弹簧，竖直放置，下端悬一质量为m的小球，开始时使弹簧为原长而小球恰好与地接触，今将弹簧上端缓慢地提起，直到小球刚能脱离地面为止，在此过程中外力作功为<img src="https://static.zxtiku.com/wangke/imgs/topic/134c7a11c1bc867c422488931694c90f.jpg"/> A. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/d1229bdc4e8aca3ef4c48e85aee5b31c.jpg"/> B. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/abe1e0c6b61f58ee898fa73bd41e1bb2.jpg"/> C. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/fb5b4d8aaed732ffbd36c2b9d49b93da.jpg"/> D. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/fcf9667925bfff75ee416ad518fba365.jpg"/> E. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/d0104be09eee08b512502abbfa0dad88.jpg"/>