“…Significant variation of BALs, including a complete disappearance, with a time scale of 1-10 year in the quasar restframe have been reported in the previous studies (e.g., Hall et al, 2011;Zhang et al, 2011Zhang et al, , 2015Filiz Ak et al, 2012Vivek et al, 2012Vivek et al, , 2014Joshi et al, 2014). The significant variation can be explained by a variation of either the ionizing power (e.g., Trevese et al, 2013) or the covering factor due to a cloud transiting the line-of-sight (e.g., Hall et al, 2011).…”