Average gross refining margin declined to $5.83 per barrel in the April-December periodIndian Oil Corp (IOC) reported nearly 91 per cent slump in quarterly profit on Wednesday, as a plunge in global crude prices took a toll on margins of oil refiners and marketing companies.The three public sector companies - IOC, Bharat Petroleum Corp and Hindustan Petroleum Corp - were forced to take a price cut in October, hurting their margins as the government wanted to cushion the impact of high crude oil prices on consumers.Net profit came in at Rs 717 crore ($100.66 million) in the quarter ended December 31, 2018, compared with Rs 7,883 crore a year earlier.Average gross refining margin declined to $5.83 per barrel in the April-December period from $8.28 per barrel in the year-ago period, the country's top refiner said on Wednesday.($1 = Rs 71.23)