The biological basis for assigning all human beings into a singe species is that there so many, even tons of living organisms living on earth. These organisms differ in their external and internal organs and so to differentiate them from them, they are being classified into kingdoms then classes then phylums then family.
m - Meter - Length.
s - Second - Time.
A - Ampere - Electric Current.
K - Kelvin - Temperature.
kg - Kilogram - Mass.
