GRE Text Completion: JiJing 1373-GRE填空机经1373题 - EM4BW799BXR4K4K0A

Historically, the depletion of soil's nitrogen has been one of the most ____________ problems faced by farmers: an essential nutrient, nitrogen is quickly leached from soil, and farmers have struggled to find ways to replenish it. A. capricious B. ubiquitous C. worrisome D. stubborn E. intractable F. unpredictable