Why is the efficiency of a machine always less than 100 percent?

Because of friction.

Explanation:

Efficiency is always less than 100% because there is always energy loss in each and every process. This loss is in the form of friction. Also, since friction is the core reason that the mechanical world exist. We even can't walk on the road if the friction is not present. Hence, we can't deny from the fact that

it also reduces the efficiency from 100%.

c) up

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answer///terminator or twilight zone;