A telephone cable is to be run from a junction point on the bank of a river 900 meters wide to an office that is located 3000 meters downstream on the opposite bank. If the cost of laying cable under water is Rs. 5 per meter and that of laying overhead on land is Rs. 4 per meter, find the point downstream where the cable is to cut across the river.
(a) 450
(b) 1800
(c) 2800
(d) 2100
(e) 2700