Question

name the three convinenient units used to measure length ranging from very short to very long value. how are they related to Si unit?

Answer 1

the SI unit of length - meter(m)

three conventional units are -

1) centimeter (cm)
1cm = 1/100 m

2)millimeter(mm)
[very short]
1mm = 1/1000 m

3)kilometer(km)
[very long]
1km = 1000m

Answer 2

milimeter is shortest unit used commonly.(nanometer is smaller than that but not used commonly.)
meter is the most commonly used unit of length.
kilometer is used to measure distance in india.For other countries it is different like in USA it's miles.(light year is longer than both but it is used to measure distance in space.)