______ is more secure than revocable one, hence the ______ is most often used in practice. According to UCP600, if an L/C is not marked as being irrevocable or not, it should be taken as ______.
【单选题】听力原文:W: Oh, I wish that bus would come! My feet are freezing! M: Have you been waiting long? W: It feels like hours --but I suppose I' ye only been here ten minutes or so, M: That' s long enough in th...