A small ball of mass 'm' is released at a height 'R' above the earth surface. The maximum depth of the ball to which it goes is R/2 inside the earth through a narrow groove before coming to rest momentarily. The groove contains an ideal spring of spring constant K and natural length R. The value of K , if R is the radius of earth and M is mass of the earth is
a) 3GMm/R³
b) 6GMm/R³
c) 9GMm/R³
d) 7GMm/R³
