If the point $(a, 0)$ is on the ray $OX^{\prime}$,is $a$ a positive number or a negative number?

(B) In a Cartesian coordinate system,the ray $OX$ represents the positive $x$-axis,where $x > 0$. The ray $OX^{\prime}$ represents the negative $x$-axis,where $x < 0$. Since the point $(a, 0)$ lies on the ray $OX^{\prime}$,the $x$-coordinate $a$ must be a negative number.