If the mean and variance of six observations \( 7, 10, 11, 15, a, b \) are \( 10 \) and \( \dfrac{20}{3} \), respectively, then the value of \( |a-b| \) is equal to:
\( 11 \)
\( 9 \)
\( 7 \)
\( 1 \)