# If $(a, a^2)$ falls inside the angle made by the lines $y=\frac{x}{2}, \;x > 0$ and $y = 3x,\; x > 0$, then $a$ belongs to :
( A ) $(-3, -\frac{1}{2})$
( B ) $(-3, \infty)$
( C ) $(\frac{1}{2}, 3)$
( D ) $(0, \frac{1}{2})$