Explanations:The measurement of distance is a fundamental concept in geography and everyday life. Different units of measurement are used depending on the scale of the distance being measured. The metric system, which is widely used around the world, includes units such as millimetres, centimetres, metres, and kilometres.
Millimetres (mm) and centimetres (cm) are used for measuring very small lengths. For example, the thickness of a book or the length of a pencil would be measured in millimetres or centimetres. These units are not suitable for measuring the distance between two cities because the distances involved are much larger.
The metre (m) is the base unit of length in the metric system. It is used for measuring medium-sized lengths, such as the height of a person or the length of a room. However, for measuring the distance between two cities, the metre is still too small a unit.
The kilometre (km) is a unit of length equal to 1,000 metres. It is commonly used for measuring long distances, such as the distance between two cities or the length of a road. The distance from Delhi to Patna, for example, is approximately 500 kilometres. This is a typical scale for intercity distances, making the kilometre the appropriate unit of measurement.
Decimetres (dm) are a unit of length equal to one-tenth of a metre. While decimetres are part of the metric system, they are not commonly used for measuring distances. They are more often used in specialized contexts, such as in scientific measurements or in certain technical fields.
In summary, the choice of unit for measuring distance depends on the scale of the distance being measured. For the distance between two cities like Delhi and Patna, the kilometre is the most appropriate unit because it provides a practical and easily understandable measure of the distance.