Answer:
A. polar regions
Explanation:
Among the last frontiers to be mapped were the polar regions. This refers to the Arctic and Antartica. These regions were difficult to access due to their remoteness and the harshness of the weather. Exploration in these regions only began in the twentieth century, and it required a lot of sacrifice on the part of the explorers. Even in modern times, these are some of the most inaccessible areas on Earth.
