After selling $17$ books for $\text{Rs. }720$, a shopkeeper says that he incurs a loss equal to the C.P. of $5$ books. Find the C.P. of a book.

1. A
   $\text{Rs. }90$
2. B
   $\text{Rs. }50$
3. C
   $\text{Rs. }60$
4. D
   $\text{Rs. }120$