9. Given $\overrightarrow { A B } \perp \overrightarrow { A C }$, $| \overrightarrow { A B } | = \frac { 1 } { t }$, $| \overrightarrow { A C } | = t$, if point $P$ is a point in the plane of $\triangle A B C$, and $\overrightarrow { A P } = \frac { \overrightarrow { A B } } { | \overrightarrow { A B } | } + \frac { \overrightarrow { A C } } { | \overrightarrow { A C } | }$, then the maximum value of $\overrightarrow { P B } \cdot \overrightarrow { P C }$ equals A. 13 B. 15 C. 19 D. 21
9. Given $\overrightarrow { A B } \perp \overrightarrow { A C }$, $| \overrightarrow { A B } | = \frac { 1 } { t }$, $| \overrightarrow { A C } | = t$, if point $P$ is a point in the plane of $\triangle A B C$, and $\overrightarrow { A P } = \frac { \overrightarrow { A B } } { | \overrightarrow { A B } | } + \frac { \overrightarrow { A C } } { | \overrightarrow { A C } | }$, then the maximum value of $\overrightarrow { P B } \cdot \overrightarrow { P C }$ equals\\
A. 13\\
B. 15\\
C. 19\\
D. 21