In a joint business, the ratio of the capitals of three partners is $5:7:9$. If the total profit earned in the business is $\text{Rs. }420$, find the share of the third partner.

1. A
   $\text{Rs. }100$
2. B
   $\text{Rs. }140$
3. C
   $\text{Rs. }180$
4. D
   $\text{Rs. }200$