A signal which can be green or red with probability 4/5 and 1/5 respectively, is received by station A and then transmitted to station B. The probability of each station receiving the signal correctly is 3/4. If the signal received at station B is green, then the probability that the original signal is green is
(a) 3/5
(b) 5/7
(c) 20/23
(d) 9/20