A is one of the 6 horses entered for a race and is to is be ridden by one of two jockeys B or C. It is 2 to 1 that B rides A, in which case all the horses are equally likely to win, if C rides A, his chance is trebled. Then the odds against his winning are ` (A) 5/13 (B) 18/5 (C) 13/18 (D) 13/5`