同种材料制成的一把椅子和一个桌子，它们的体积之比为1：4，则它们的质量之比为_____， 密度之比为_______．
1∶4；1∶1
小题1:Children need time to do things by t_______(他们自己)
小题2: Rodert and his friends walked home together in s________(沉默)
小题3: Could you pass on the m________(消息) to Alice?
小题4:Marcia wants to have a s _________(惊讶) party for her mom.
小题5: How l_________(幸运的) you are!
互补品的需求交叉弹性是（）
A．正数
B．负数
C．零
D．1
