Optimization Question - Offshore Oil Rig

An offshor oil rig is located at Point A, which is 10km from Point B directly oon shore. An oil pipeline is to be constructed from Point A to C & then to an oil refinery at Point D. What is the minimum cost of the pipeline if it costs $9,000/km to lay under the sea & $3,000/km to lay on land. Length B to D is 50km