Savannas can be found anywhere but must be located in a tropical climate to thrive.
The U.S.A. has a very moderate climate, and for the most part is the same overall. Thus, a savanna wouldn't be able to exist in the U.S.A without the tropical climate, which means all 12 months have temperatures around 64 degrees Fahrenheit, and there are only two seasons: wet and dry.
As opposed to the U.S.A's Moderate Springs, Blazing summers, chilly falls, and cold winters (may fluctuate depending on the state), a savanna wouldn't be able to thrive.
Your answer is D: they exist only in the tropics.
~Hope this helps!
