Concept:Map scale relates a distance on the map to the corresponding actual ground distance.Explanation:The map scale is given as 2 cm=1 km.Map distance between the towns is 20 cm.Divide the map distance by the scale length: 20÷2=10.Multiply by the ground equivalent: 10×1 km=10 km.Therefore, the actual ground distance is 10 km.Answer:C. 10 km