There has been great enthusiasm in the United States for reducing fossil fuel dependence by increasing production of biofuels from crops such as corn and switchgrass, but this (i)____________ about biofuels' potential should be (ii)____________ by a realistic appraisal of the costs and challenges of biofuel production. A. forbearance B. tempered C. exuberance D. delineated E. obduracy F. exacerbated