1 cm = 10 mm
1 m = 100 cm = 1000 mm
Again, 1 km = 1000 m = 100000 cm = 10000000 mm.
Hence, 1 mm is smaller than 1 cm, 1 cm is smaller than 1 m, and 1 m is smaller than 1 km, i.e.,
1 millimetre < 1 centimetre < 1 metre < 1 kilometre