HEY!!
Answer:
A. friction
Explanation:
Now if we obtain output work same as input energy then it is 100% efficient.
but this is only possible when we have no energy loss while it is not possible due to many reasons:
one of the main cause of energy loss is friction
A machine can never be 100% efficient because some work is always lost due to the lack of materials or equipment that would convert work by 100%. but so far, no machine has got 100% efficiency.
So when the machine are working there must be a friction between its working parts due to which the out-input energy is lost in different parts and hence we can never be 100% efficient machine.
HOPE IT HELPS!!
