Let $10$ vertical poles standing at equal distances on a straight line,subtend the same angle of elevation $\alpha$ at a point $O$ on this line and all the poles are on the same side of $O$. If the height of the longest pole is $h$ and the distance of the foot of the smallest pole from $O$ is $a$,then the distance between two consecutive poles is:

• A
  $\frac{h \cos \alpha - a \sin \alpha}{9 \sin \alpha}$
• B
  $\frac{h \sin \alpha + a \cos \alpha}{9 \sin \alpha}$
• C
  $\frac{h \cos \alpha - a \sin \alpha}{9 \cos \alpha}$
• D
  $\frac{h \sin \alpha - a \cos \alpha}{9 \cos \alpha}$