Acivilization on a vast, open plain with rich resources is likely to be:
a. suffering under a lack of contact with the outside world,
b. protected from invasions.
c. domesticating goats and chickens.
d. vulnerable to attack from warring civilizations.
which is

Humid continental climate is found in the interior of continents between the 30 and 60 degrees latitude, but usually above the 40 degree line.   40 degrees north of the equator means less direct sunlight and therefore less warmth.   this climate is mainly found in the northern hemisphere since there are no major landmasses in the southern hemisphere.  if you live in northern indiana, you know all about humid continental because that is the climate type you have!

I'm pretty sure it's c. extremely gradual but not 100% sure though.