Answer:
c. Buildings that were built with logs from mountains about 50 miles away
Step-by-step explanation:
Buildings made of logs from mountains about 50 miles away are the evidence of traders over distances because people need a shelter at a place for trading and they form buildings with logs.
Traders migrate to other places for trading nearby 30-50 miles, they make the same designed building for shelter and move ahead after their trade is done and leave the building as it is.
Hence, the correct answer is "c".