For a reconnaissance survey, it is necessary to obtain vertical aerial photographs of a terrain at an average scale of $1: 13000$ using a camera. If the permissible flying height is assumed as $3000 \mathrm{~m}$ above a datum and the average terrain elevation is $1050 \mathrm{~m}$ above the datum, the required focal length (in $\mathrm{mm}$ ) of the camera is
- $100$
- $150$
- $125$
- $200$