A bag contains 6 white and 4 black balls. Two balls are drawn at random. The probability that they are of the same colour, is

$\begin {array} {1 1} (a)\; \frac{1}{15} & \quad (b)\;\frac{2}{5} \\ (c)\;\frac{4}{15} & \quad (d)\;\frac{7}{15} \end {array}$

$(d)\frac{7}{15}$