The mean and variance of \( 10 \) observations are \( 9 \) and \( 34.2 \), respectively. If \( 8 \) of these observations are \( 2, 3, 5, 10, 11, 13, 15, 21 \), then the mean deviation about the median of all the \( 10 \) observations is:
\( 6 \)
\( 7 \)
\( 5 \)
\( 4 \)