If for $p \neq q \neq 0$, then function $f ( x ) = \frac { \sqrt [ 7 ] { p ( 729 + x ) } - 3 } { \sqrt [ 3 ] { 729 + q x } - 9 }$ is continuous at $x = 0$, then\\
(1) $7 p q f ( 0 ) - 1 = 0$\\
(2) $63 q f ( 0 ) - p ^ { 2 } = 0$\\
(3) $21 q f ( 0 ) - p ^ { 2 } = 0$\\
(4) $7 p q f ( 0 ) - 9 = 0$