In the expansion of \( \left(9x-\frac{1}{3\sqrt{x}}\right)^{18},\ x>0 \), if the term independent of \( x \) is \( (221)k \), then \( k \) is equal to:
\( 84 \)
\( 78 \)
\( 168 \)
\( 198 \)