Why do we calculate theoretical mechanical advantage rather than the real mechanical advantage?

Question

a
It's too hard to calculate the real mechanical advantage because so much energy is transferred as thermal energy
b
It's too hard to calculate the real mechanical advantage because so much energy is lost
c
It's an easier equation to use
d
The theoretical is better because it lets us know everything that will in reality happen with the machines ---which is never why we use oil on bike chains

Answer 1

c. It's an easier equation to use