C seems to be the best. D talks of professor's who are commenting on college economics but that area of group of professor's are not specified i mean to say suppose some are part time/ full time /speciliZed in one subject, also the case possible is they are commenting on college economics we dont certainly know whats their level of education whether they are phd masters so its just out of context and doesnt affects the conclusion.
Practice Practice and practice...!!
If my reply /analysis is helpful-->please press KUDOS
If there's a loophole in my analysis--> suggest measures to make it airtight.