What is the key external force that allows a man to push a wheelbarrow

Friction

Explanation:

The key and most important external forces that allows a man to push a wheelbarrow is frictional force.

Frictional force is a force that opposes motion and will bring it to a halt.

The reason why motion is possible is due to frictional force.

Without friction, a body will move and continue in motion infinitely. No one will be able to control motion.

the answer would most likely be c, because if it's a comical piece of information, it may likely not be true, if it's a site that allows anybody to edit the content, it could mean a five year old is typing the info, and if it's a tv show produced by a celeb, it may have a biased opinion.

i think the answer is a, hope this : )