A uniform solid sphere of mass $M$ and radius ' $a$ ' is surrounded by a concentric uniform thin spherical shell of mass $0.5 \mathrm{M}$ and radius $1.5 \mathrm{a}$. The gravitational potential energy of a unit mass kept at a distance of $2.5 \mathrm{a}$ from the center is

• Correct answer
  A

  $\frac{-3 \mathrm{GM}}{5 \mathrm{a}}$
• B

  $\frac{3 \mathrm{GM}}{5 \mathrm{a}}$
• C

  $\frac{2 \mathrm{GM}}{5 \mathrm{a}}$
• D

  $\frac{-2 \mathrm{GM}}{5 \mathrm{a}}$

Hence, the correct answer is option: 'A'.