Editor
The middle terms in the expansion $(3x-\frac{x^{3}}{6})^{9}$ is
Difficulty: Easy
A: 
$-\frac{27}{26}x^{27}$
B: 
$-\frac{21}{17}x^{17}$
C: 
$\frac{25}{2}x^{6}$
D: 
$-\frac{21}{16}x^{19}$