18. come (run) across (偶然)碰到 ; get sth. across 使人理解;领会 19. act as充当,担任 ;act out 表演(对话、故事); act on(upon) 对...起作用 20 catch sb. in the act of doing sth. 抓住某人干某事; take action 采取行动 21 be...