A string of length 90 cm is fastened to two points A and B at the same level 60 cm apart. A ring weighing 120 g is slided on the string. A horizontal force P is applied to the ring such that it is in equilibrium vertically below B. The value of P is ________.

A)
40 g
B)
60 g
C)
80 g
D)
100 g

Correct Answer :   80 g