The starting motor of a car is turning too slowly, and the mechanic has to decide whether to replace the motor, the cable, or the battery. The car's manual says that the 12V battery should have no more than 0.020Ω internal resistance, the motor no more than 0.200Ω resistance and the cable any more than 0.040Ω resistance. The mechanic turns on the motor and measures 11.4V across the battery, 3.0 V across the cable, and a current of 50 A. Which part is defective?