已知 $\stackrel{\rightharpoonup }{AB}\perp \stackrel{\rightharpoonup }{AC},\left|\stackrel{\rightharpoonup }{AB}\right|=\frac{1}{t},\left|\stackrel{\rightharpoonup }{AC}\right|=t$，若 $P$点是 $△ABC$所在平面内一点，且 $\stackrel{\rightharpoonup }{AP}=\frac{\stackrel{\rightharpoonup }{AB}}{\left|\stackrel{\rightharpoonup }{AB}\right|}+\frac{4\stackrel{\rightharpoonup }{AC}}{\left|\stackrel{\rightharpoonup }{AC}\right|}$ ，则 $\stackrel{\rightharpoonup }{PB}·\stackrel{\rightharpoonup }{PC}$的最大值等于（    ）

A．

13

B．

15

C．

19

D．

21