下列事件是必然事件的是（　　）

    Many people travel during the holiday season but do not make sure that their houses and homes ____ (protect). Crimes go up ____ (rapid) during the winter and summer holiday seasons. Here are some things that you should keep in mind when you go on holiday.

    Always give ____ (strange) the feeling that you are at home. Have the snow ____ (clean) off your stairs or out of your driveway during the winter season. You might ask someone to park his/her car in ____ (you) driveway.

    Tell your newspaper deliverer that you are not at home. ____ (have) a pile of newspapers and other mail on your doorsteps tells people that you are not at home, so you could also have a neighbor ____ a relative get your mail every day.

    Fix a timer(定时器) in some of your rooms ____ turns lights on and ____ during different time of the day. Some TVs also come with a timer that you could set to be turned on during certain time. Have motion(运动) sensitive lights outside your house that keep thieves away ____ you are not at home.

“梅隆拉响汽笛，胡佛敲起钟，华尔街发出信号，美国朝地狱里冲。”这首20世纪30年代流行的民谣出现的时代背景是

生物体在结构上具有明显的层次性：例如：单细胞层次的生物：    ____     ；组织层次的多细胞生物：    ____     ；器官层次的多细胞生物：    ____     ；系统层次的多细胞生物：      ____   。