If you break the words down on C, <u>Conservation</u> means: the prevention of wasteful resources. <u>Tillage</u>: preparation for growing crops. And clearly, they did not prevent or till the correct way, because: "The Dust Bowl was caused by several economic and agricultural factors, including federal land policies, changes in regional weather, farm economics and other cultural factors. After the Civil War, a series of federal land acts coaxed pioneers westward by incentivizing farming in the Great Plains."
So clearly the answer is D, because of all the erosion and drought, (and more causes) there was a dust bowl.
Ang mga hadlang sa kalakalan ay mga paghihigpit na sapilitan ng pamahalaan sa internasyonal na kalakal, na karaniwang binabawas ang pangkalahatang kahusayan sa ekonomiya.
