Ram put some buttons on the table. There were 4 blue, 7 red, 3 black and 6 white buttons in all. All of a sudden, a cat jumped on the table and knocked out one button on the floor. What is the probability that the button on the floor is blue?
Options:
(a) `7/20`
(b) `3/5`
(c) `1/5`
(d) `1/4`