The author mentions the estimates of "One writer" primarily in order to A. indicate in a general way the size that a carbon tax must be for it to be effective B. provide the most accurate information available about the most practical size for a carbon tax C. suggest that the target recommended by the 1988 Toronto Conference is an unrealistic one D. undermine the argument that a carbon tax would provide incentives for user's to achieve emissions reductions E. show how the size of an effective carbon tax can be calculated