For two points $\mathrm { A } ( 2,0,1 ) , \mathrm { B } ( 3,2,0 )$ in coordinate space, if the coordinates of a point on the $y$-axis that is equidistant from both points is $( 0 , a , 0 )$, what is the value of $a$? [2 points]\\
(1) 1\\
(2) 2\\
(3) 3\\
(4) 4\\
(5) 5