The focal length of a converging lens is the distance.

Positive

: For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens (for example a convex lens), the focal length is positive and is the distance at which a beam of collimated light will be focused to a single spot.

B) raising the temperature and reducing the volume of the gas.|-/ 

Ais the answer hope this