what is the correct answer; a, b, c, or d?

Mining scenario A mining company owns two mines, each of which produces three grades (high, medium, and low) of ore. The company has a contract to supply a smelting company with at least 12 tons of high-grade ore, at least 8 tons of medium-grade ore, and at least 24 tons of low-grade ore. Each hour of operation, mine I produces 6 tons of high-grade ore, 2 tons of medium-grade ore, and 4 tons of low-grade ore. Each hour of operation, mine 2 produces 2 tons of high-grade ore, 2 tons of medium-grade ore, and 12 tons of low-grade ore. It costs $200 per hour to operate mine 1 and $160 per hour to operate mine 2 . How many hours should each mine be operated so as to meet the contractual obligations at the lowest total operating cost? Use this sensitivity report to answer the questions below. What constraints are binding (at that optimal solution)? Low-grade High-grade and Medium-grade Medium-grade High-grade and Low-grade