In an opaque bag, there are $5$ red balls, $5$ white balls, and $5$ yellow balls. The balls are the same except for their colours. Take out one ball from the bag. What is the probability that you take out a yellow ball? 
选项为:A:$\dfrac{1}{2}$  B:$\dfrac{1}{3}$  C:$\dfrac{1}{4}$  请回答其中一个选项.