In my textbook there's an exercise in which I must decide if a sentence is correct. There's this: 在客厅里坐着一位老人. In the answers they say it's incorrect because "zai4" shouldn't be there. Is it really a mistake? I always thought "zai4" was optional in sentences like (zai4) + somewhere + li3/ shang4/ xia4/ zuo3 bian/ you4 bian/ qian2/ hou4.