Atoms $_{7} X^{n},{ }_{8} Y^{D}$ and ${ }_{9} Z^{17}$ are such that ${ }_{8} Y$ is an isobar of ${ }_{7} X$ and atom ${ }_{9} Z^{17}$ is isotone of ${ }_{8} Y .$ Mass no. of $X$ and no. of neutrons in $Y$ are respectively :
(a) 8,8
(b) 17,7
(c) 9,8
(d) 16,8