Final Answer:
The people of the Andes trade their crops through 'vertical trade' with villages at different elevations, thus the correct option is a.
Step-by-step explanation:
Vertical trade is a type of trading system in which people from different elevations in the Andes Mountains trade their crops with each other. This type of trading system has been used by the people of the Andes for centuries and it is still used today. In vertical trade, the people living at higher elevations usually trade their crops with people living at the lower elevations. This is because the higher elevations experience cooler temperatures and harsher climates which makes it difficult to grow crops. The lower elevations, on the other hand, experience warmer temperatures and much more suitable climate for growing crops. Therefore, the people from the lower elevations are able to produce more crops and can trade with the people living in the higher elevations.
This type of trading system is beneficial for both parties involved. The people living in the higher elevations are able to get access to the crops they need to survive, while the people in the lower elevations are able to make a profit from trading their crops. This type of trading system is also beneficial for the environment, as it reduces the need for transportation and delivery of goods between the two locations.
In conclusion, the people of the Andes trade their crops through 'vertical trade' with villages at different elevations. This type of trading system is both beneficial for the people involved and also for the environment.