13. Among the following propositions about lines $l$, $m$ and planes $\alpha$, $\beta$, the true proposition is
A. If $l \subset \beta$ and $\alpha \perp \beta$, then $l \perp \alpha$.
B. If $l \perp \beta$ and $\alpha \parallel \beta$, then $l \perp \alpha$.
C. If $l \perp \beta$ and $\alpha \perp \beta$, then $l \parallel \alpha$.
D. If $\alpha \cap \beta = m$ and $l \parallel m$, then $l \parallel \alpha$.

If the point $(a, 0)$ $(a > 0)$ is a center of symmetry of the graph of the function $y = 2\tan\left(x - \frac{\pi}{3}\right)$, then the minimum value of $a$ is
A. $\frac{\pi}{6}$
B. $\frac{\pi}{3}$
C. $\frac{\pi}{2}$
D. $\frac{4\pi}{3}$

In the interval $( - 2 \pi , 0 )$, the function $f ( x ) = \sin \left( \frac { 1 } { x ^ { 3 } } \right)$
(A) never changes sign
(B) changes sign only once
(C) changes sign more than once, but finitely many times
(D) changes sign infinitely many times

In the interval $( 0,2 \pi )$, the function $\sin \left( \frac { 1 } { x ^ { 3 } } \right)$
(a) never changes sign
(b) changes sign only once
(c) changes sign more than once, but finitely many times
(d) changes sign infinitely many times.

In the interval $( 0,2 \pi )$, the function $\sin \left( \frac { 1 } { x ^ { 3 } } \right)$
(a) never changes sign
(b) changes sign only once
(c) changes sign more than once, but finitely many times
(d) changes sign infinitely many times.