Increase their vocabulary and comprehension
I would go with:
A theme is best written as statement because a theme is main thing of a story, the author could also use a statement in a story to make it seem more alive or more real.
I Hope my answer worked for you :)
Explanation : Q: In what ways do the military practices described by Marco Polo reflect the influence of the Steppe environment on the Mongols?
Answer: A: They could get better due to the ability to practice on the main land.