Answer:
[a] Friction is a force.
[c] An unbalanced force is necessary to cause a non-moving object to start moving.
Explanation:
Friction is a force that exerts oppositely from an objects motion, mostly by its surface. The smoothness and roughness of the surface depends on how much friction it has, friction is a type of force.
Unbalanced forces are what causes objects that aren't moving to move. For example, wind causes a kite to sway in the sky. Wind, in this situation, is the source of unbalance force.
