# Light planet is revolving around a very massive star in a circular orbit of radius 'r' with a period of revolution T. On what power of 't' will the square of period depend if the gravitational force of attraction between planet and the star is proportional to $r^{\large\frac{-5}{2}}$
$(a)\;T^2 \; \alpha \; r^{\large\frac{+7}{2}} \quad (b)\;T^2 \; \alpha \; r^{\large\frac{-7}{2}} \quad (c)\; T^2 \; \alpha \; r^{\large\frac{3}{2}} \quad (d)\;T^2 \; \alpha \; r^{\large\frac{-3}{2}}$