Question

a machine can never be 100% efficient because some work is always lost due to which factor?
friction
gravity
static
input force

Explanation:

In machines, friction causes energy - loss as heat and other forms, reducing efficiency. Gravity is a general force not the main cause of work - loss in machines. "Static" is not a force causing work - loss here. Input force is what powers the machine, not a cause of inefficiency.

Answer:

A. friction