A model test is to be conducted in a water tunnel using a 1: 20 model of a submarine, which is to travel at a speed of 12 km/h deep under sea surface. The water temperature in the tunnel is maintained, so that its kinematics viscosity is half that of sea water. At what speed is the model test to be conducted to produce useful data for the prototype?
a. 12 km/h b. 240 km/h
c. 24 km/h d. 120 km/h