A contractor wishes to determine a suitable combination of manual labour and machine work for the excavation of a multistory construction. For every cum of excavation, 3 man-hours are needed; or 0.2 machine-hour. Costs involved are Rs. 20 per man-hour and Rs. 500 per machine-hour. The total quantity of excavation is estimated to be 4,000 cum. The optimal quantity of excavation to be done manually for minimum total cost will be (a) 1500 cum (b) 1800 cum (c) 2250 cum (d) 2500 cum