I believe that they used the Oregon Trail
I believe that the correct answer is D.)Animal Species
Animals reproduce , breed , and affect the agriculture.
Look up the word agriculture , you will see how my answer makes more sense.
My reason being is mainly because everything basically lives off of animals and if the are affected by anything and in any way start to decrease , the population will also.
I really hope that this helps you out a lot.
<span>first one is either C or D and o nthe second one i think that when the reinasance times started, so c improve</span>
