16. Plot the functions
$$
y=\log _{3} x, y=\log _{x} 3, y=\log _{\frac{1}{3}} x, y=\log _{x} \frac{1}{3}
$$

on the same coordinate system. Then on the positive $x$-axis direction, there are ( ) intersection points.
(A) 2
(B) 3
(C) 4
(D) 5
(E) 6
16. D.

Let $t=\log _{3} x(x>0)$. Then $\frac{1}{t}=\log _{x} 3, -t=\log _{\frac{1}{3}} x, -\frac{1}{t}=\log _{x} \frac{1}{3}$.
Notice that, the graphs of the functions
$$
y=t, y=\frac{1}{t}, y=-t, y=-\frac{1}{t}
$$

have five intersection points.
Therefore, the functions
$$
y=\log _{3} x, y=\log _{x} 3, y=\log _{\frac{1}{3}} x, y=\log _{x} \frac{1}{3}
$$

have five intersection points on the positive x-axis.