Answer the following question given this paragraph: 

where m is the mass of the system. Experiments have shown that acceleration is exactly inversely proportional to mass, just as it is exactly linearly proportional to the net external force.


Q: Experiments have shown that acceleration is exactly inversely proportional to mass, just as it is exactly linearly proportional to what?

 Choices:

- net internal force

- speed

- kinetic energy

- net external force

A:
net external force
