Organisms have very few parts that are not useful to the organism. This is because

Those parts have become vestigial

Explanation:

Organisms have few parts that are not useful to them because those parts in them have become vestigial.

When an organism has a part is no more using, it relates to the evolutionary history of such organism.

r = ρ x length / area  

ρ = r x area / length  

ρ is in ohm-meters  

in basic units, it is m³kg/s³a²  

i think this is it.