— Could you tell me ___________?
— Sorry, I don't know. I will call you when he comes back.
A：when will your brother come back
B：when your brother will come back
C：how your brother will come back
D：how will your brother come back
根的长长与根尖中的____有关；根吸收水分的主要部位是____，因为该处生有大量的____。
为了铸造金属工件，事先用密度为ρ1的实木材料制成木模，木模的质量为1.8kg．再用密度为ρ2的合金铸造30个这样的工件，这些合金的总质量为648kg，则木模与合金材料密度之比为( )
