In the given figure, a circle is inscribed in $$\triangle$$PQR, such that it touches the sides PQ, QR and RP at points D, E, F, respectively. If the lengths of the sides PQ = 18 cm. QR = 13 cm and RP = 15 cm, then find the length of PD.

A

10 cm

B

8 cm

C

15 cm

D

12 cm