This question examines the knowledge points of length units.

Measure the thickness of CD, coins, bank cards, the length of screws, paper clips, erasers, chalk, etc. , generally in millimeters.

The unit of length refers to the basic unit for regulating the length. Its international unit is "meter" (symbol "m"), and commonly used units are millimeter (mm), centimeter (cm), decimeter (dm), kilometer (km), meter (m), micron (micron) and nanometer (nm). The basic transformation relationship is as follows:

1 km =1000m =10000m =100000mm =100000mm.