$$1\rm m=100\rm cm$$ and $$1\rm km=1000m$$; the correct ratio is $$100:1000$$. 
(the number of $$\rm cm$$ in $$1\rm m$$):(the number of $$\rm m$$ in $$1\rm km$$)$$=$$． 
  - $$100:1000$$ 
  - $$1000:100$$ 
  - $$1:100$$ 
  - $$100:1$$ 