It was A. Densely wooded forests.
In total, the Black Death has killed about 450 million down to 350-370 million in the 14th century.
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Weather will be the primary reason. Despite its huge size. the real habitable area is probably less than 10% of its geographical size. Even Southern Ontario is deemed by many I know to be "too cold to live". 90% of Ontario and Quebec are simply inhabitable. I also personally think the entire Manitoba and Saskachewan are not suitable for human habitation unless we can create a way to simply hibernate during the 6 month winter (why there are so few people in Montana and North Dakota?) The fact that Montreal are thrive as a vibrant metropolis is already sort of a miracle and we don't see many such large cities with such severe weather. The only other case I can think of is probably Moscow.
Answer- Frogs back feet are webbed to help them swim.
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