A spherical ball of mass 20 kg is stationary at the top of a hill of height 100 m. It rolls down a smooth surface to the ground, then climbs up another hill of height 30 m and finally rolls down to a horizontal base at a height of 20 m above the ground. The velocity attained by the ball is :

• A

  $40\sqrt{\frac{5}{7}}\mathrm{m}/\mathrm{s}$

• B

  20m/s

• C

  40 m/s

• D

  $10\sqrt{30}\mathrm{m}/\mathrm{s}$