When all those present(到场者)-- he began his lecture.A.sit B.s

D were seated
A sit 是不及物动词.这里是不是该用 sit down
B set 作动词时是放置物品或是某物,某人处于某种状态,没有就座的意思.
C seat 的特点是作动词时为及物动词,后面必须有宾语或用其被动语态
如：Be seated,please.请坐.（很礼貌的用法）
seat sb.某人就座（有安排,使的意味 ） seat yourself ,seat him