Answer:
From 1820 to 1860, the number of cities with over 10,000 people increased.
Step-by-step explanation:
This is the answer that best explains the changes that took place from 1820 to 1860 as recorded in this map. In 1820, we can see around a dozen cities with a population of over 10,000 people. This was a significant population at the time. However, we can see that by the 1860s, the number of cities with this population had greatly increased. This was most likely a consequence of immigration and industrialization.