单选题 Anything that is dropped from a height falls towards the center of the earth because of the pull of______.
【正确答案】 C
【答案解析】解析：gravity n.重力，地心引力；严肃：严重(性)。weight n．重量，重力；重压，负担；重要性，价值。attraction n.吸引，吸引力；具有吸引力的人或东西。density n.密度；密集，稠密。