During gym class, a boy climbs a vertical rope and temporarily stops half way between the ground below and the ceiling above. Consider the following forces: (a) gravity, (b) normal force, (c) friction, and (d) tension. At that moment, ignoring any effects due to the surrounding air, which of these forces are acting on the boy?
O Both a and c
O Both a and d
O All of these forces act on the boy
O Both a and b