According to the passage, which one of the following is a cause of the dwindling stock of heirloom crop varieties mai...

Where can the support for this answer be found? Initially, I chose A and realized that is never stated in the passage. I then chose C thinking that in the first paragraph " tended to base their research and their thinking solely on agricultural methods brought to North America from elsewhere. But given the intensity with which these non–North American methods seek to maximize the land's output for economic reasons". Is C wrong because it does not say anywhere directly that the reason for not choosing heirloom is due to lower yields? I wanted to pick E but couldn't find any support in the passage. I see in paragraph 3 where it says "penetration of the world market economy into small agricultural communities, both native and non-native, has discouraged small-scale, internal food production in favor of income-generating work."