Answer the following questions related to gravitation:
a. (i) State the universal law of gravitation and write its mathematical form.
b. (ii) If the distance between two point masses is reduced to one-third of its original value while their masses remain unchanged, explain qualitatively and quantitatively how the gravitational force between them is affected.
c. (iii) A small object is placed exactly at the centre of a thin, hollow, uniform spherical shell of mass. Explain why the net gravitational force acting on the object is zero, even though the shell attracts the object from every side.