Answer the following question given this paragraph: 

The mechanical advantage of a simple machine such as a pulley is the factor by which the machine changes the force applied to it. The ideal mechanical advantage of a machine is its mechanical advantage in the absence of friction. All machines must overcome friction, so the ideal mechanical advantage is always somewhat greater than the actual mechanical advantage of the machine as it is used in the real world.


Q: What is something that all machines must overcome?

 Choices:

- stress

- work

- temperature

- friction

A:
friction
