answer: #Person2# explains to #Person1# what is combined transportation and why it's necessary.
documents: #Person1#: What's combined transportation?
  #Person2#: If the goods are to be transshipped from one means of transportation to another during the course of the entire voyage, it's called'combined transportation'.
  #Person1#: Why is it necessary?
  #Person2#: It's simpler and cheaper to arrange multi-model combined transportation.