Two lenses of power $-15\text{D}$ and $+5\text{D}$ are in contact with each other. The focal length of the combination is:

1. A
   $-20\text{cm}$
2. B
   $-10\text{cm}$
3. C
   $+20\text{cm}$
4. D
   $+10\text{cm}$